Tab key must pass through. This is important if you want to do keyboard navigation through this control.

Fix problem: 
Set hotkey [TheSameHOTKEY], then mousedown on delete button, then set hotkey again  [TheSameHOTKEY], then this hotkey do not work.

I could find no way to make the feature work, so I have pulled it
from the code. I have also inserted asserts to warn library users
who would attempt to use illegal characters in user defaults keys
in the future. In short, you want your user defaults keys to be
something identifier-like.