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
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.
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)