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.

icons/bb-oled-symbolic.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <path d="M 8 1.3 L 13.5 3.4 V 7.6 C 13.5 11.2 11.2 13.6 8 14.7 C 4.8 13.6 2.5 11.2 2.5 7.6 V 3.4 Z"
+        fill="none" stroke="#2e3436" stroke-width="1.3" stroke-linejoin="round"/>
+  <path d="M 5.6 8.1 L 7.3 9.8 L 10.6 6.2" fill="none" stroke="#2e3436" stroke-width="1.3" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>