app: add an "Open as Image" button to the brushes dialog

It was probably forgotten to add in the first place (the patterns
dialog also has one).

(cherry picked from commit fc609d12dd)

app/widgets/gimpbrushfactoryview.c

@@ -35,6 +35,7 @@
 
 #include "gimpbrushfactoryview.h"
 #include "gimpcontainerview.h"
+#include "gimpeditor.h"
 #include "gimpmenufactory.h"
 #include "gimpspinscale.h"
 #include "gimpviewrenderer.h"
@@ -151,6 +152,10 @@ gimp_brush_factory_view_new (GimpViewType     view_type,
 
   editor = GIMP_CONTAINER_EDITOR (factory_view);
 
+  gimp_editor_add_action_button (GIMP_EDITOR (editor->view),
+                                 "brushes", "brushes-open-as-image",
+                                 NULL);
+
   gtk_box_pack_end (GTK_BOX (editor->view), factory_view->spacing_scale,
                     FALSE, FALSE, 0);
   gtk_widget_show (factory_view->spacing_scale);