Replaced legacy GTK type macro blocks with G_DECLARE_* declarations for GtkXText, CustomList, and SexySpellEntry, and aligned the GtkXText parent field naming across GTK2/GTK3 guards.

Updated RemoteObject to use G_DECLARE_FINAL_TYPE and the parent_instance field naming convention

src/fe-gtk/xtext.h

@@ -137,7 +137,7 @@ struct _GtkXText
 #if HAVE_GTK3
 	GtkWidget parent_instance;
 #else
-	GtkWidget widget;
+	GtkWidget parent;
 #endif
 
 	xtext_buffer *buffer;