GTK native dialogs hang with idle (zero) timer

We need to do idle processing, and the documented way to do so is with a QTimer set to 0 milliseconds. Which seems to work peachy, until we try to open a file dialog.

At that point, the main window goes gray (indicating a transient child window has been created), but nothing more. Pressing escape seems to abort things, and things get responsive again.

Digging around, I can find that this happens when it tries to open GTK's file dialog. It runs `gtk_dialog_run()`, which starts a nested main loop. This keeps firing our idle worker, but seems to be blocking something else that is critical.

Issue does not happen if we force Qt to use its own file dialog.