Unified threading for long operations

[novhack]

For the UX it is very important to never block the main thread with long operations which could make Pixelorama unresponsive. Especially Windows are very quick to mark an app as non-responsive.

I propose that we could make a global threading class/node which would dispatch a potentially long task to a separate thread and which would display a blocking unified progress dialog.

I implemented something like this for exporting but I think that especially import functionality should be handled in a thread too.