Instead of creating/destroying a window whenever the drawin is attached/detached
from some screen, a window is now created when the drawin is created and it
isn't destroyed until the drawin is garbage collected.
Since we can now safely assume that a drawin always has a associated window,
this should make it possible to simplify some stuff.
Signed-off-by: Uli Schlachter <psychon@znc.in>