There isn't much point in the screen property of a drawin because the geometry
already defines on which screen the drawin is.
The biggest chunk of changes from this is due to removing the functions
drawin_detach and drawin_attach which where called when the screen property was
set/unset.
The code from drawin_detach is moved into drawin_set_visible and drawin_wipe.
drawin_attach is moved into drawin_init.
Signed-off-by: Uli Schlachter <psychon@znc.in>
08cd44dda7