wmaker: fix clearing of window attribute that was not saved properly

wmaker: fix clearing of window attribute that was not saved properly

As reported by Nerijus Baliunas, there was a problem when unchecking an
attribute in the Window Inspector and saving it. The original code was
meant to save an attribute that is being checked by user, but not one that
is explicitly unchecked, which means than although it looked ok when using
the "Apply" button, it was not remembered when restarting the application.

In continuation to the clean-up started in the previous patches, this one
is updating the Window Inspector to display 2 check-boxes, one read-only on
the left, displaying the state requested by the application, and a second
one which makes use of the new Tri-State button in WINGs to let the user
specify if he wants to force-on, force-off, or leave as-is the attribute.

The saving to the property list is then updated to take into account this
new 3-state when saving to the file, so relaunching the application will
remember correctly the user choice.

Reported-by: Nerijus Baliunas <nerijus@users.sourceforge.net>
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>