This adds gears.screen which contains a wrapper around
screen.connect_signal("added", func) that also calls the callback function for
each screen that already exists. This is added in gears.screen so that it can
also be used from e.g. wibox, if needed. Feel free to move this elsewhere if
that's a bad idea (I'm not really convinced of it).
Signed-off-by: Uli Schlachter <psychon@znc.in>