Add a "Cancel" button for Zerebus soundfont loading & audio export
Zerebus sound fonts (e.g. Salamander Grand piano) can be very large. Loading them will use up a lot of memory. If user is on a memory starved system, then will cause thrashing and possibly hang the computer and may result in a crash (e.g. https://musescore.org/en/node/71216). In case the user discovers that the loading is starting to hang, it would be nice to have a "Cancel" button so user can abort the loading and free up the memory.