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.