app: add gegl:focus-blur to Filters -> Blur

gegl:focus-blur blurs the image around a focal point.  It can be
used to create fake depth-of-field effects.

Add a prop-gui constructor which uses a FOCUS controller to control
the focus geometry.

(cherry picked from commit 60d4d25b93)

menus/image-menu.xml.in

@@ -696,6 +696,7 @@
       <menuitem action="plug-in-reset-all" />
       <separator />
       <menu action="filters-blur-menu" name="Blur">
+	<menuitem action="filters-focus-blur" />
 	<menuitem action="filters-gaussian-blur" />
 	<menuitem action="filters-mean-curvature-blur" />
 	<menuitem action="filters-median-blur" />