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_darwin.go

@@ -0,0 +1,58 @@
+package dialog
+
+import (
+	"github.com/sqweek/dialog/cocoa"
+)
+
+func (b *MsgBuilder) yesNo() bool {
+	return cocoa.YesNoDlg(b.Msg, b.Dlg.Title)
+}
+
+func (b *MsgBuilder) info() {
+	cocoa.InfoDlg(b.Msg, b.Dlg.Title)
+}
+
+func (b *MsgBuilder) error() {
+	cocoa.ErrorDlg(b.Msg, b.Dlg.Title)
+}
+
+func (b *FileBuilder) load() (string, error) {
+	return b.run(false)
+}
+
+func (b *FileBuilder) save() (string, error) {
+	return b.run(true)
+}
+
+func (b *FileBuilder) run(save bool) (string, error) {
+	star := false
+	var exts []string
+	for _, filt := range b.Filters {
+		for _, ext := range filt.Extensions {
+			if ext == "*" {
+				star = true
+			} else {
+				exts = append(exts, ext)
+			}
+		}
+	}
+	if star && save {
+		/* OSX doesn't allow the user to switch visible file types/extensions. Also
+		** NSSavePanel's allowsOtherFileTypes property has no effect for an open
+		** dialog, so if "*" is a possible extension we must always show all files. */
+		exts = nil
+	}
+	f, err := cocoa.FileDlg(save, b.Dlg.Title, exts, star)
+	if f == "" && err == nil {
+		return "", ErrCancelled
+	}
+	return f, err
+}
+
+func (b *DirectoryBuilder) browse() (string, error) {
+	f, err := cocoa.DirDlg(b.Dlg.Title)
+	if f == "" && err == nil {
+		return "", ErrCancelled
+	}
+	return f, err
+}