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.

main/TopBar.tscn

@@ -39,6 +39,20 @@ theme_type_variation = &"HighlightLarge"
 text = "DEBUG"
 vertical_alignment = 1
 
+[node name="UpdateLabel" type="Label" parent="." unique_id=1959221258]
+visible = false
+layout_mode = 0
+offset_left = 1096.0
+offset_top = 12.0
+offset_right = 1432.0
+offset_bottom = 50.0
+mouse_filter = 1
+theme_type_variation = &"HighlightMedium"
+text = "Version 0.0.0 is available"
+horizontal_alignment = 2
+vertical_alignment = 1
+text_overrun_behavior = 3
+
 [node name="SyncButton" type="Button" parent="." unique_id=1228313556]
 layout_mode = 0
 offset_left = 1450.0