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.

Makefile

@@ -13,9 +13,10 @@ schemas:
 build: schemas
 
 install: build
-	mkdir -p $(INSTALL_DIR)/lib $(INSTALL_DIR)/schemas
+	mkdir -p $(INSTALL_DIR)/lib $(INSTALL_DIR)/schemas $(INSTALL_DIR)/icons
 	cp extension.js prefs.js metadata.json stylesheet.css $(INSTALL_DIR)/
 	cp lib/*.js $(INSTALL_DIR)/lib/
+	cp icons/*.svg $(INSTALL_DIR)/icons/
 	cp $(SCHEMA_DIR)/*.xml $(SCHEMA_DIR)/gschemas.compiled $(INSTALL_DIR)/schemas/
 	@echo "Installed to $(INSTALL_DIR)"
 	@echo "Reload GNOME Shell (Alt+F2, r, Enter on X11; log out/in on Wayland), then run:"
@@ -25,7 +26,7 @@ uninstall:
 	rm -rf $(INSTALL_DIR)
 
 pack: schemas
-	gnome-extensions pack --force --extra-source=lib -o dist .
+	gnome-extensions pack --force --extra-source=lib --extra-source=icons -o dist .
 
 clean:
 	rm -f $(SCHEMA_DIR)/gschemas.compiled