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])

tests/fakes/ui_tour.gd

@@ -77,6 +77,7 @@ static func make_server() -> FakeHttpTransport:
 	server.respond_json("/music/all", SOUNDTRACKS)
 	server.respond_json("/music/all/order", SOUNDTRACKS)
 	server.respond_json("/music/list", [])
+	server.respond_json("/download", "1.0.0")
 	var characters: Array[String] = character_names()
 	server.respond_json("/character/list", characters)
 	for character_name: String in characters:
@@ -243,6 +244,13 @@ func run(on_state: Callable) -> void:
 	sync_popup.close()
 	await _settle()
 
+	var top_bar: TopBar = _node("TopBar")
+	top_bar.show_latest_version("99.10.10")
+	await _settle()
+	await on_state.call("update_available", null)
+	top_bar.show_latest_version("1.0.0")
+	await _settle()
+
 	var statistics_view: StatisticsView = _node("StatisticsView")
 	_main.show_tab(&"statistics")
 	await _settle()