app: add new action "view-scroll-center" with shortcut Shift+J

The zoom focus discussion on IRC suggests that everybody is annoyed
about centering behavior (or lack thereof), so here is a way to
explicitly center the image witout zooming.

menus/image-menu.xml.in

@@ -294,6 +294,7 @@
         <separator />
         <menuitem action="view-rotate-other" />
       </menu>
+      <menuitem action="view-scroll-center" />
       <separator />
       <menuitem action="view-shrink-wrap" />
       <menuitem action="view-fullscreen" />