background: when a file changes, remove it from the image cache

MetaBackgroundCache doesn't itself use file watches, so when a image
that we are monitoring changes we need to purge it from the cache,
so that when we load it again we get the new image.

https://bugzilla.gnome.org/show_bug.cgi?id=710756

js/ui/background.js

@@ -300,6 +300,8 @@ const Background = new Lang.Class({
         let signalId = this._cache.connect('file-changed',
                                            Lang.bind(this, function(cache, changedFile) {
                                                if (changedFile == filename) {
+                                                   let imageCache = Meta.BackgroundImageCache.get_default();
+                                                   imageCache.purge(changedFile);
                                                    this.emit('changed');
                                                }
                                            }));