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-patches/01-xinitthreads.patch

@@ -0,0 +1,20 @@
+--- a/dlgs_linux.go
++++ b/dlgs_linux.go
+@@ -1,8 +1,9 @@
+ package dialog
+
+-// #cgo pkg-config: gtk+-3.0
++// #cgo pkg-config: gtk+-3.0 x11
+ // #include <gtk/gtk.h>
+ // #include <stdlib.h>
++// #include <X11/Xlib.h>
+ // static GtkWidget* msgdlg(GtkWindow *parent, GtkDialogFlags flags, GtkMessageType type, GtkButtonsType buttons, char *msg) {
+ // 	return gtk_message_dialog_new(parent, flags, type, buttons, "%s", msg);
+ // }
+@@ -13,6 +14,7 @@
+ import "unsafe"
+
+ func init() {
++	C.XInitThreads()
+ 	C.gtk_init(nil, nil)
+ }