changed: explicitly cast to int to quell compiler warning
fixed: initialize in declaration order
changed: split off IPlayer callback to a separate file

helps keep rebuilds down if IPlayer.h is touched.
brings it down from 149 files rebuilt to 30 (149 for new file).
thus no spurious rebuilds was found but callback should change
less
changed: split off window id's to separate file

Key.h rebuilds from 291 to 129 (232 for new file) => 60 spurious
rebuilds killed
changed: remove unnecessary includes