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-general-symbolic.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <g fill="none" stroke="#2e3436" stroke-width="1.3" stroke-linecap="round">
+    <line x1="3.5" y1="2" x2="3.5" y2="14"/>
+    <line x1="8" y1="2" x2="8" y2="14"/>
+    <line x1="12.5" y1="2" x2="12.5" y2="14"/>
+  </g>
+  <circle cx="3.5" cy="5.5" r="1.7" fill="#2e3436"/>
+  <circle cx="8" cy="10.5" r="1.7" fill="#2e3436"/>
+  <circle cx="12.5" cy="7" r="1.7" fill="#2e3436"/>
+</svg>