Well, that OCR needle isn't working out so great, as it seems
to match when it shouldn't:
https://openqa.fedoraproject.org/tests/119217#step/_graphical_wait_login/5
So let's try another approach. Ditch the OCR needle and have a
function for checking we're at a clean desktop. It does the
normal needle match, but if we're on GNOME, it also tries
hitting alt+f1 and seeing if we're at the overview; if so, it
hits alt+f1 again (to go back to the desktop) and returns.
