Add Clear cache to the settings

A new row, "Downloaded pictures and songs", shows how much space the cached
character pictures, covers and downloaded songs take, with a Clear cache
button. Clearing deletes them, but keeps the songs the current quiz uses,
the settings, config.ini and the saved quiz. The characters are loaded
again at once; covers come back when they are shown.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_clear_cache.gd.uid

@@ -0,0 +1 @@
+uid://da5wh1me3wncq