Add a button in Settings that opens the config folder

Shows the folder with config.ini, the saved settings and the cached
characters and songs in the system's file manager.

Closes #45

tests/integration/test_offline_mode.gd

@@ -124,6 +124,8 @@ func test_server_features_are_off_and_the_folder_is_shown() -> void:
 	assert_str(_top_bar.offline_label.text).is_equal("Offline: Music")
 	assert_bool(_settings.select_server_button.disabled).is_true()
 	assert_bool(_settings.add_to_database_button.disabled).is_true()
+	# The config folder holds the cached characters, which are still used offline
+	assert_bool(_settings.open_config_folder_button.disabled).is_false()
 
 
 func test_picking_a_folder_on_the_statistics_tab_goes_to_the_quiz() -> void: