fix excessive loading of keys file caused by xmodmap

xmodmap (and other tools) trigger MappingNotify events. a single xmodmap
expression such as "keycode comma = comma semicolon" might trigger 4 or 5
MappingNotify events. loading the keys file on each of them is quite
unefficient.

fluxbox now uses a (250ms) timer which is reset upon further events.
use function fluxbox::findScreen()