Add missing import for Gio

I got GitHub Copilot to guess what the import should be, since I didn't know, and I don't think it was in the original code (as posted).

src/textual_paint/wallpaper.py

@@ -88,6 +88,7 @@ def set_wallpaper(file_loc: str, first_run: bool = True):
     if desktop_env in ["gnome", "unity", "cinnamon"]:
         uri = "'file://%s'" % file_loc
         try:
+            from gi.repository import Gio
             SCHEMA = "org.gnome.desktop.background"
             KEY = "picture-uri"
             gsettings = Gio.Settings.new(SCHEMA)