Move the fullscreen API from GimpDisplayShell to GimpImageWindow

Also remove GimpDisplayShell's "window_state" member. Use
gtk_widget_get_toplvel() to get to the GimpImageWindow when we need a
display shell's fullscreen state.

app/display/gimpimagewindow.h

@@ -51,6 +51,9 @@ GType         gimp_image_window_get_type           (void) G_GNUC_CONST;
 
 GimpDisplay * gimp_image_window_get_active_display (GimpImageWindow *window);
 
+void          gimp_image_window_set_fullscreen     (GimpImageWindow *window,
+                                                    gboolean         fullscreen);
+gboolean      gimp_image_window_get_fullscreen     (GimpImageWindow *window);
 
 
 #endif /* __GIMP_IMAGE_WINDOW_H__ */