Eliminate the Ubuntu Unity integration

[ferdnyc]

I honestly don't think the Unity progress-bar integration was working even on Ubuntu anymore, but turns out now it's also causing crashes on Fedora 33 when Python tries to import gi.

This PR rips out the Unity integration, and moves the export-progress signals from main_window to export as they're now only used locally.

It also removes an aboutToQuit signal connection I had added, which is causing double unsaved-project prompts on exit if you say "No" to the first one.

[ferdnyc]

Going to merge this as well. There's some possibility that the Unity integration is still useful to people, so its removal might impact them, but it's a non-critical piece of functionality. (Worst case, a progress bar is no longer displayed on the application icon during export — that's the only thing lost here. Assuming it even worked.)

We could certainly restore it, with a better implementation. But preventing full-on crashes (on some non-Ubuntu systems) during export trumps the convenience of a progress bar.