Add a popup that displays when a song is taking too long to load

[Kermalis]

This would show up after loading for x seconds (probably should customize the time) and would ask if loading would like to be cancelled, while also explaining what happens when you cancel loading (time bar and midi exporting should be disabled, and track viewer will have some issues, as well as large memory consumption until the song is changed).

Alternatively we can just try to load another song or dispose all memory we used while loading.