Show in the top bar when a newer version is available

At start and after switching server, the game asks the server for the newest
MusicPlayer release and shows "Version X is available" in the top bar when it
is newer than this one.

tests/fakes/main_window_test_suite.gd

@@ -31,6 +31,7 @@ func before_test() -> void:
 	_server.respond_json("/music/all/order", ["Chrono Trigger", "Final Fantasy VI"])
 	_server.respond_json("/music/list", [])
 	_server.respond_json("/character/list", [])
+	_server.respond_json("/download", "1.0.0")
 	_server.respond_bytes("/music/rand", mp3)
 	_server.respond_bytes("/music/addQue", PackedByteArray())
 	_server.respond_json("/music/info", SONG_INFO[0])