Handle pointer grabbing abortion

Unmapping the parent of the *input only* window aborts the pointer
grabbing and makes us stuck in `wait_event`, so we need to rely on the
leave notify event generated when the grabbing stops.
The focus as multiple ways of being invalid

We shall not try to add a child to the *none* or *pointer root* windows.
Destroying can't fail

The only possible error for window destroying is when the window is
already destroyed, so we shall not die for that (it seems that the
unchecked version have no effect though).