Update menu_base implementation, assets and documentation

locale/en.properties

@@ -32,10 +32,6 @@ menu/save/hint = Click a slot to save the game; a used slot is overwritten after
 menu/play/hint_warming = The games' mods are being prepared in the background — starting will be quick afterwards
 menu/play/hint_ready = The games' mods are ready, starting takes a few seconds
 menu/load/legacy_group = Saves without a game
-menu/load/legacy_title = Save without a game
-menu/load/legacy_text = This save was made before game collections existed. Choose the game it belongs to — the file moves into that game's saves.
-menu/load/legacy_ok = Move and load
-menu/load/legacy_no_games = There is no ready game to assign the save to.
 menu/load/diff_title = The game has changed
 menu/load/diff_intro = The game's composition changed since this save.
 menu/load/diff_added = Added (starts from scratch):