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/cocoa/dlg.h

@@ -0,0 +1,39 @@
+#include <objc/NSObjCRuntime.h>
+
+typedef enum {
+	MSG_YESNO,
+	MSG_ERROR,
+	MSG_INFO,
+} AlertStyle;
+
+typedef struct {
+	char* msg;
+	char* title;
+	AlertStyle style;
+} AlertDlgParams;
+
+#define LOADDLG 0
+#define SAVEDLG 1
+#define DIRDLG 2 // browse for directory
+
+typedef struct {
+	int mode; /* which dialog style to invoke (see earlier defines) */
+	char* buf; /* buffer to store selected file */
+	int nbuf; /* number of bytes allocated at buf */
+	char* title; /* title for dialog box (can be nil) */
+	void** exts; /* list of valid extensions (elements actual type is NSString*) */
+	int numext; /* number of items in exts */
+	int relaxext; /* allow other extensions? */
+} FileDlgParams;
+
+typedef enum {
+	DLG_OK,
+	DLG_CANCEL,
+	DLG_URLFAIL,
+} DlgResult;
+
+DlgResult alertDlg(AlertDlgParams*);
+DlgResult fileDlg(FileDlgParams*);
+
+void* NSStr(void* buf, int len);
+void NSRelease(void* obj);