Added GtkAdjustment helper accessors for value/upper/lower/page size/page increment to encapsulate GTK3 accessors and GTK2 struct fields.

Updated xtext adjustment logic to use the helpers in adjustment updates and rendering paths, eliminating direct field access in scroll/render flows.

src/fe-gtk/xtext.h

@@ -128,7 +128,11 @@ typedef struct {
 
 struct _GtkXText
 {
+#if HAVE_GTK3
+	GtkWidget parent_instance;
+#else
 	GtkWidget widget;
+#endif
 
 	xtext_buffer *buffer;
 	xtext_buffer *orig_buffer;