widget: remove grab key filter

The widget currently filters out last key press from grab key sequence
if it's not a modifier key. But this will prevent nested usage of ungrab
combinations such as shift+f12.

https://bugzilla.redhat.com/show_bug.cgi?id=889962

This will also make the following obsolete:
https://bugzilla.redhat.com/show_bug.cgi?id=846005