debounce repeated keypresses on hold

[brenopacheco]

This resolves https://github.com/baskerville/sxhkd/issues/156 https://github.com/baskerville/sxhkd/issues/298 by discarding keypress events that happen a very small amount of time after a keyrelease event is received the same (debounce_ms).

I know this solution is not optimal.

Tested with -d 10 and it seems to work fine, though it does not work with something like

alt + Return
alt + @Return