wxGTK: do not freeze/thaw the main frame window

On certain configurations, this is problematic, because windows
sometimes are not thaw()'ed correctly, leading to an interface
that doesn't update. This is a workaround that simply disables all
freeze/thaw on the main frame. I haven't noticed any performance
issues.

Originally committed to SVN as r5373.

aegisub/src/frame_main.h

@@ -93,6 +93,11 @@ public:
 	agi::scoped_ptr<agi::Context> context;
 
 private:
+    // XXX: Make Freeze()/Thaw() noops on GTK, this seems to be buggy
+#ifdef __WXGTK__
+    void Freeze(void) {}
+    void Thaw(void) {}
+#endif
 	void cmd_call(wxCommandEvent& event);
 
 	/// DOCME