Refresh refactoring, frontend/UIManager side

This changes the refresh API. The handling is only modified slightly. There now exists a queue of refresh requests. Requests are merged when overlapping. The only method to trigger a repaint and/or a refresh is now UIManager:setDirty(). Everything else is gone.
Note that the region was only applied in one branch of the test because the PDF 'flipping' mode relied on the refresh triggered here to refresh the full screen. If that's still the case, it'd probably be more elegant to actually 'fix' the flipping mode instead of perpetuating my quick workaround ;).