Introduce new common GUI key_ helper functions

These will transform from a key event to a key combination integer, and
then from a key combination integer to a human-readable description.
Port Configurator UI window to use key_combo_desc

This actually reduces the code size quite a bit.
Bug fix for exception when unsetting keys

This bug has probably been there since the 1.0 beta 12 release, but it
is sure going to be fixed now.
Fix for key_event_combo()

Returns correct value for modifier keys now.
Implement new key binding from Configurator

This uses a combination of keyPressEvent/keyReleaseEvent to accomplish
the goal, so that it's as easy as possible to bind keys.