Fix hacked GTK3/GLFW crash by vendoring a patched sqweek/dialog

The built hacked binary segfaults on Linux the moment a native GTK3
dialog (file/folder browse) opens while a GLFW/OpenGL context is live:
sqweek/dialog's Linux backend never calls XInitThreads() before
gtk_init(), and GLFW's own X11 backend installs a process-wide Xlib
error handler that collides with GTK's.

Vendor a pinned copy of github.com/sqweek/dialog (New BSD) into
modding/sqweek-dialog/ so it can be patched like every other vendored
source here: sqweek-dialog-patches/01-xinitthreads.patch adds the
missing XInitThreads() call, and hacked-patches/01-use-patched-dialog.patch
adds a go.mod replace directive pointing hacked's own sqweek/dialog
dependency at that local copy. build-hacked.sh now stages both hacked/
and sqweek-dialog/ into a scratch modding/build/ directory and applies
their patch series before compiling, rather than building hacked/ in
place; .gitignore picks up that new scratch directory.

Also expand modding/README.md: number its existing sections and add a
worked "Example: Editing a message" walkthrough covering Static World
Data, the Messages window's clipboard-only text editing, saving the
Mod Location, and the absolute-path requirement for run.sh's mod-folder
argument.

modding/sqweek-dialog/dlgs.go

@@ -0,0 +1,146 @@
+// Package dialog provides a simple cross-platform common dialog API.
+// Eg. to prompt the user with a yes/no dialog:
+//
+//     if dialog.MsgDlg("%s", "Do you want to continue?").YesNo() {
+//         // user pressed Yes
+//     }
+//
+// The general usage pattern is to call one of the toplevel *Dlg functions
+// which return a *Builder structure. From here you can optionally call
+// configuration functions (eg. Title) to customise the dialog, before
+// using a launcher function to run the dialog.
+package dialog
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrCancelled is an error returned when a user cancels/closes a dialog.
+var ErrCancelled = errors.New("Cancelled")
+
+// Cancelled refers to ErrCancelled.
+// Deprecated: Use ErrCancelled instead.
+var Cancelled = ErrCancelled
+
+// Dlg is the common type for dialogs.
+type Dlg struct {
+	Title string
+}
+
+// MsgBuilder is used for creating message boxes.
+type MsgBuilder struct {
+	Dlg
+	Msg string
+}
+
+// Message initialises a MsgBuilder with the provided message.
+func Message(format string, args ...interface{}) *MsgBuilder {
+	return &MsgBuilder{Msg: fmt.Sprintf(format, args...)}
+}
+
+// Title specifies what the title of the message dialog will be.
+func (b *MsgBuilder) Title(title string) *MsgBuilder {
+	b.Dlg.Title = title
+	return b
+}
+
+// YesNo spawns the message dialog with two buttons, "Yes" and "No".
+// Returns true iff the user selected "Yes".
+func (b *MsgBuilder) YesNo() bool {
+	return b.yesNo()
+}
+
+// Info spawns the message dialog with an information icon and single button, "Ok".
+func (b *MsgBuilder) Info() {
+	b.info()
+}
+
+// Error spawns the message dialog with an error icon and single button, "Ok".
+func (b *MsgBuilder) Error() {
+	b.error()
+}
+
+// FileFilter represents a category of files (eg. audio files, spreadsheets).
+type FileFilter struct {
+	Desc       string
+	Extensions []string
+}
+
+// FileBuilder is used for creating file browsing dialogs.
+type FileBuilder struct {
+	Dlg
+	StartDir string
+	Filters  []FileFilter
+}
+
+// File initialises a FileBuilder using the default configuration.
+func File() *FileBuilder {
+	return &FileBuilder{}
+}
+
+// Title specifies the title to be used for the dialog.
+func (b *FileBuilder) Title(title string) *FileBuilder {
+	b.Dlg.Title = title
+	return b
+}
+
+// Filter adds a category of files to the types allowed by the dialog. Multiple
+// calls to Filter are cumulative - any of the provided categories will be allowed.
+// By default all files can be selected.
+//
+// The special extension '*' allows all files to be selected when the Filter is active.
+func (b *FileBuilder) Filter(desc string, extensions ...string) *FileBuilder {
+	filt := FileFilter{desc, extensions}
+	if len(filt.Extensions) == 0 {
+		filt.Extensions = append(filt.Extensions, "*")
+	}
+	b.Filters = append(b.Filters, filt)
+	return b
+}
+
+// SetStartDir specifies the initial directory of the dialog.
+func (b *FileBuilder) SetStartDir(startDir string) *FileBuilder {
+	b.StartDir = startDir
+	return b
+}
+
+// Load spawns the file selection dialog using the configured settings,
+// asking the user to select a single file. Returns ErrCancelled as the error
+// if the user cancels or closes the dialog.
+func (b *FileBuilder) Load() (string, error) {
+	return b.load()
+}
+
+// Save spawns the file selection dialog using the configured settings,
+// asking the user for a filename to save as. If the chosen file exists, the
+// user is prompted whether they want to overwrite the file. Returns
+// ErrCancelled as the error if the user cancels/closes the dialog, or selects
+// not to overwrite the file.
+func (b *FileBuilder) Save() (string, error) {
+	return b.save()
+}
+
+// DirectoryBuilder is used for directory browse dialogs.
+type DirectoryBuilder struct {
+	Dlg
+	StartDir string
+}
+
+// Directory initialises a DirectoryBuilder using the default configuration.
+func Directory() *DirectoryBuilder {
+	return &DirectoryBuilder{}
+}
+
+// Browse spawns the directory selection dialog using the configured settings,
+// asking the user to select a single folder. Returns ErrCancelled as the error
+// if the user cancels or closes the dialog.
+func (b *DirectoryBuilder) Browse() (string, error) {
+	return b.browse()
+}
+
+// Title specifies the title to be used for the dialog.
+func (b *DirectoryBuilder) Title(title string) *DirectoryBuilder {
+	b.Dlg.Title = title
+	return b
+}