Fix actions that shouldn't happen while autosaving

[manongjohn]

While reviewing #1332, I noticed there is a potential for overlapping save operations under certain scenarios which could potentially result in file corruption. Additionally, #1332 suggests an undo operation was performed while an autosave was in progress. This could cause a crash when it tries to access something that may no longer exist.

I've added logic to internally flag when a save is in progress so that any additional calls to save while actively saving will wait until the flag is cleared.

Scenarios that should be covered by this fix

• Timed autosave + suspended autosave due to active drawing or something
• Timed autosave + manual save by user
• Timed autosave + Undo/Redo

NOTE: It's hard to duplicate overlapping saving issues naturally. I mostly regression tested for normal saving and autosaving operations as well as a simple saving error (file locked) and ensuring T2D didn't completely freeze up.

[artisteacher]

Thanks for tackling this!