document or remove QApplication.processEvents() call from queue logic

[redshiftzero]

Followup question from #404 (comment):

Looks like the only incoming signals this thread needs to handle are the download_file_thread.started and main_thread.started signals, which only start this processing loop, right? When the loop is running, there shouldn't need to be any handling of incoming signals right? I'm curious if this really needs to be called. The docs say:

The event loop is started with a call to exec(). Long-running operations can call processEvents() to keep the application responsive.

But we're already in a separate thread from the application, so we don't need to worry about the application not being able to respond, I think.

I thought that this call is needed for processing events from queued signal/slot connections. Let's dig and check if that's true or not: if the call is needed, we should document via comments in the code why it's necessary, else we should remove