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/README.md

@@ -0,0 +1,35 @@
+# dialog
+Simple cross-platform dialog API for go-lang
+
+# examples
+    ok := dialog.Message("%s", "Do you want to continue?").Title("Are you sure?").YesNo()
+
+Creates a dialog box titled "Are you sure?", containing the message "Do you want to continue?",
+a "Yes" button and a "No" button. Returns true iff the dialog could be displayed and the user
+pressed the "Yes" button.
+
+    filename, err := dialog.File().Filter("Mp3 audio file", "mp3").Load()
+
+Creates a file selection dialog allowing the user to select a .mp3 file. The absolute path of
+the file is returned, unless an error is encountered or the user cancels/closes the dialog.
+In the latter case, `filename` will be the empty string and `err` will equal `dialog.Cancelled`.
+
+    filename, err := dialog.File().Filter("XML files", "xml").Title("Export to XML").Save()
+
+Asks the user for a filename to write data into. If the user selects a file which already exists,
+an additional dialog is spawned to confirm they want to overwrite the existing file.
+
+    directory, err := dialog.Directory().Title("Load images").Browse()
+
+Asks the user for a directory.
+
+# platform details
+* OSX: uses Cocoa's NSAlert/NSSavePanel/NSOpenPanel classes
+* Win32: uses MessageBox/GetOpenFileName/GetSaveFileName (via package github.com/TheTitanrain/w32)
+* Linux: uses Gtk's MessageDialog/FileChooserDialog (via cgo; requires gtk3 development packages)
+
+# build
+```
+go mod download
+go build
+```