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/example/simple/main.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/sqweek/dialog"
+)
+
+func main() {
+	/* Note that spawning a dialog from a non-graphical app like this doesn't
+	** quite work properly in OSX. The dialog appears fine, and mouse
+	** interaction works but keypresses go straight through the dialog.
+	** I'm guessing it has something to do with not having a main loop? */
+	dialog.Message("%s", "Please select a file").Title("Hello world!").Info()
+	file, err := dialog.File().Title("Save As").Filter("All Files", "*").Save()
+	fmt.Println(file)
+	fmt.Println("Error:", err)
+	dialog.Message("You chose file: %s", file).Title("Goodbye world!").Error()
+	dialog.Directory().Title("Now find a dir").Browse()
+}