app, menus: add gegl:variable-blur to Filters -> Blur

gegl:variable-blur applies a blur with a per-pixel radius
controlled by a mask.

(cherry picked from commit 721f2d8c27)

menus/image-menu.xml.in

@@ -701,6 +701,7 @@
 	<menuitem action="filters-median-blur" />
 	<menuitem action="filters-pixelize" />
 	<menuitem action="filters-gaussian-blur-selective" />
+        <menuitem action="filters-variable-blur" />
 	<placeholder name="Motion">
 	  <menuitem action="filters-motion-blur-circular" />
 	  <menuitem action="filters-motion-blur-linear" />