Preferences: write preferences on exit

This avoids the issue where user added shortcuts or quick favourites
are lost on exit.

source/blender/windowmanager/intern/wm_init_exit.c

@@ -52,6 +52,7 @@
 #include "BLO_writefile.h"
 #include "BLO_undofile.h"
 
+#include "BKE_blendfile.h"
 #include "BKE_blender.h"
 #include "BKE_blender_undo.h"
 #include "BKE_context.h"
@@ -469,6 +470,10 @@ void WM_exit_ext(bContext *C, const bool do_python)
       WM_event_remove_handlers(C, &win->modalhandlers);
       ED_screen_exit(C, win, WM_window_get_active_screen(win));
     }
+
+    if (U.runtime.is_dirty) {
+      BKE_blendfile_userdef_write_all(NULL);
+    }
   }
 
   BLI_timer_free();