Fix crashes when pressing tab

EDIT: I decided to combine the 3 MRs:

Fix crash when the window disappears

The redraw procedure was allowed to run after a window was destroyed

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Don't let dead widgets be clicked

Fixes crash when a widget would be free'd but a click event would still try to access it.

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Keep keyboard visible when buttons are pressed

This fixes momentary clicks which hide the keyboard, causing the button press never to be released.

It's still possible to get keys stuck by drag-and-dropping away from the original key.

Addresses the root cause of #18 (closed)