Fix crash when window is closed while thread running

[samuel-w]

Fixes #684

I only opened the issue to confirm that it wasn't a weird setup issue, since I remember this not crashing in the past.

Opening and closing the window is so much faster now, as an upside.

Check to make sure this doesn't reintroduce #340, doesn't in my testing.

[samuel-w]

Takes up about 6-10MB more memory when window closed.

[m3nu]

Sounds good. You want to do a PR for those changes, since you already have them working? I'll test on macOS.

[samuel-w]

This is the pull request.
Here is what I have tested, all working:

• Closing/opening window with repository window open
• Running check and opening/closing window
• Creating backup from tray and opening/closing
• Creating backup from window and opening/closing

[samuel-w]

Another solution is making the parent of the threads VortaApp.

Downsides:
The main window will be slower to open.
Upsides:
Cleaner solution
Doesn't undo an earlier commit
Saves tiny amount of memory
Easier to test

[samuel-w]

Went with the alternate method of fixing it, see comment above. Might undo it later, since I'm still on the fence about it.

Closing and opening the window clears the messages. Going to fix that. Would be easier to keep the messages using original method.

[m3nu]

Closing and opening the window clears the messages. Going to fix that. Would be easier to keep the messages using original method.

I also think that keeping the window state would have benefits.

[m3nu]

Tested on macOS. Didn't see any issues. Actually nice to keep the window state when reopening it.

[samuel-w]

I've decided to both keep the window state, and correctly set the borg thread parents. Technically the second one is not needed, but its nice to have.

[m3nu]

Merged. I like it how window state, logs, etc are preserved. Good change!