some changes to focus handling.

some changes to focus handling.

most interesting is the change in focus_fallback, which means that it won't
call xsetinput focus in some cases, potentially reducing flicker and stuff.
also potentially producing bugs? heh.

the screen.c focus fallback code doesn't seem to need special cases anymore,
which is really good, if that is really the case.

move the focus_tried stuff out of event.c into focus.c, where it seems to
belong.