Add live wallpaper source folders and bundle preferences page icons

Live Wallpaper page gains a "Wallpaper Folders" list (mirroring the
static rotation one) plus an "Available Media" list of the videos/GIFs
found inside them to pick from, backed by a new live-wallpaper-folders
GSettings key and a generalized wallpaperSource.js extension scan.

The preferences page-switcher icons (General, Rotation, Live Wallpaper,
OLED Protection, About) were showing as broken-image placeholders since
they depended on exact symbolic icon names being present in the user's
system icon theme. Bundle a small set of custom SVG icons under icons/
and register them via Gtk.IconTheme.add_search_path so they always
resolve regardless of the system theme.

lib/settingsKeys.js

@@ -13,6 +13,7 @@ export const SettingsKey = Object.freeze({
 
     LIVE_WALLPAPER_ENABLED: 'live-wallpaper-enabled',
     LIVE_WALLPAPER_PATH: 'live-wallpaper-path',
+    LIVE_WALLPAPER_FOLDERS: 'live-wallpaper-folders',
     LIVE_WALLPAPER_MUTED: 'live-wallpaper-muted',
     LIVE_WALLPAPER_PLAYBACK_RATE: 'live-wallpaper-playback-rate',
     LIVE_WALLPAPER_PAUSE_ON_BATTERY: 'live-wallpaper-pause-on-battery',
@@ -32,3 +33,7 @@ export const SettingsKey = Object.freeze({
 export const IMAGE_EXTENSIONS = Object.freeze([
     '.jpg', '.jpeg', '.png', '.webp', '.bmp', '.tiff', '.tif', '.gif',
 ]);
+
+export const LIVE_WALLPAPER_EXTENSIONS = Object.freeze([
+    '.gif', '.mp4', '.webm', '.mkv', '.mov', '.avi', '.m4v', '.ogv',
+]);