Fixes for sloppy and mouse focus mode

This fixes a bunch of issues with sloppy and mouse focus modes. In particular alt+tab'ing is now working correctly, mouse focus mode is again able to unfocus when the mouse is not over a window.

The only undesirable behavior that's left is that with sloppy and mouse focus when the overview is closed and the mouse moved to a not-focused window, the focus will stay there until the mouse leaves the current window. This is hard to avoid without also breaking the alt+tab feature. We'd need to keep track which kind of grab leaves the focus on a specific window and with which grab the focus is supposed to move somewhere else again.