windowManager: Replace custom shader with builtin ClutterEffects

windowManager: Replace custom shader with builtin ClutterEffects

While modal dialogs were attached to the parent's titlebar, it
made sense to leave the top of the parent window at full color.
With the new position of modal dialogs, it makes more sense to dim
the entire parent window, so we can use a combination of Clutter's
BrightnessContrast- and DesaturateEffect instead of our own custom
shader.

https://bugzilla.gnome.org/show_bug.cgi?id=674499