MonitorManager: return the new backlight after changing

Modify the interface of ChangeBacklight to return the new value,
to account for rounding to HW limits.

https://bugzilla.gnome.org/show_bug.cgi?id=706729
window: ignore skip-taskbar hint on parentless dialogs

Dialogs that don't have a parent should not be skip-taskbar,
otherwise they get lost and there is no way to recover them
(because they're not autoraised when activating the parent),
but toolkits and applications set the hint anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=673399