Guarded GTK2-only GtkObject includes in the xtext header for GTK2 builds only.

Added a shared cleanup helper plus GTK2 destroy/GTK3 dispose handlers with correct parent chaining for cleanup parity across GTK versions.
Updated class initialization to register GTK3 dispose or GTK2 destroy handlers with appropriate object class setup.

src/fe-gtk/xtext.h

@@ -21,6 +21,9 @@
 #define ZOITECHAT_XTEXT_H
 
 #include <gtk/gtk.h>
+#if !HAVE_GTK3
+#include <gtk/gtkobject.h>
+#endif
 #include <cairo.h>
 #include "xtext-color.h"