Add gimp_image_window_get_active_display()

Should probably be refactored into something involving a GimpContext,
but it's good enough as intermediate step.

app/display/gimpimagewindow.c

@@ -27,6 +27,7 @@
 #include "widgets/gimpmenufactory.h"
 #include "widgets/gimpuimanager.h"
 
+#include "gimpdisplayshell.h"
 #include "gimpimagewindow.h"
 
 #include "gimp-intl.h"
@@ -169,3 +170,14 @@ gimp_image_window_destroy (GtkObject *object)
 
   GTK_OBJECT_CLASS (parent_class)->destroy (object);
 }
+
+
+/*  public functions  */
+
+GimpDisplay *
+gimp_image_window_get_active_display (GimpImageWindow *window)
+{
+  g_return_val_if_fail (GIMP_IS_IMAGE_WINDOW (window), NULL);
+
+  return GIMP_DISPLAY_SHELL (window)->display;
+}

app/display/gimpimagewindow.h

@@ -47,5 +47,8 @@ struct _GimpImageWindowClass
 
 GType         gimp_image_window_get_type           (void) G_GNUC_CONST;
 
+GimpDisplay * gimp_image_window_get_active_display (GimpImageWindow *window);
+
+
 
 #endif /* __GIMP_IMAGE_WINDOW_H__ */