Fix MIDI Loading Failing Whilst a MIDI is playing (#23339)

Fix MIDI Loading Failing Whilst a MIDI is playing (#23339)

SS14-1148 Fix MIDI Loading Failing Whilst a MIDI is playing

The behaviour of the button event handling did some wonky
async handling that got PJB swearing repeatedly in the contributor
VC.

Improve switching MIDI songs by:

0. Add a bool that tracks if we're currently waiting for the MIDI file
browser to terminate. Use this bool to short-circuit the
MidiFileButtonOnPressed function, ensuring you don't have to close
a morbillion file windows if you spam-clicked the  button or forgot
you'd opened the window.
1. Remove a four-year-old hack involving waiting 100ms to load a MIDI
after trying to stop the last MIDI, because _the rot consumes_ or some shit