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>

main/main_window.gd

@@ -39,6 +39,7 @@ func _ready() -> void:
 	top_bar.settings_pressed.connect(settings_popup.open)
 	settings_popup.server_changed.connect(quiz.on_server_changed)
 	settings_popup.characters_changed.connect(quiz.character_select_popup.load_characters)
+	settings_popup.clear_cache_requested.connect(clear_cache)
 	settings_popup.visibility_changed.connect(_on_settings_popup_visibility_changed)
 
 	sync_popup.sync_started.connect(_on_sync_started)
@@ -63,6 +64,14 @@ func _ready() -> void:
 	show_about_if_new_version()
 
 
+## Deletes the cached character pictures and covers. The characters are loaded again at
+## once, so the character select has its pictures; covers come when they are shown.
+func clear_cache() -> void:
+	quiz.character_select_popup.character_cache.clear()
+	statistics_view.covers.clear()
+	quiz.character_select_popup.load_characters()
+
+
 # The quiz finds a soundtrack's cover by the cover times in the soundtrack list
 func _remember_cover_times(soundtracks: Array[Soundtrack]) -> void:
 	var times: Dictionary = {}