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.
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Cosmetic
Fix #3: don't die if destroying fails (thanks to @baskerville)