This commits adds awesome.register_xproperty(). This allows lua code to register
arbitrary X11 properties with awesome which will then watch these properties.
Whenever such a property is changed on a client or drawin, we will emit the
xproperty::name signal.
This also adds window:get_xproperty(name) and window:set_xproperty(name, value)
which allows to mess with properties.
Signed-off-by: Uli Schlachter <psychon@znc.in>
62e2dee4ba