Allow selecting animated GIFs as live wallpapers

The playback pipeline already typefinds by content and decodes GIFs
via GStreamer's decodebin like any other video, but the preferences
file picker filtered to video/* MIME types only, hiding .gif files.
Widen the filter to include image/gif and *.gif, add an "All files"
fallback, and update the related copy/docs to mention GIF support.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RDqbdjsiisSU7CbQke4g8

lib/liveWallpaper.js

@@ -6,7 +6,10 @@ import * as Main from 'resource:///org/gnome/shell/ui/main.js';
 import {SettingsKey} from './settingsKeys.js';
 
 /**
- * Plays a video file as a looping animated background.
+ * Plays a video or animated GIF file as a looping animated background.
+ * `playbin` typefinds the source by content rather than extension, so GIFs
+ * are decoded through GStreamer's own GIF element (from the "good" plugin
+ * set) and handled exactly like any other video stream below this point.
  *
  * Mutter embeds its own private copy of Clutter, so GStreamer video sinks
  * that hand back a Clutter actor from a *different* Clutter instance (e.g.