Add a trough in window controls

[Aditya V]

As discussed with Christopher Horton earlier ( https://plus.google.com/109509247327281928543/posts/aB4cgVyZACG ), it would be nice if there would be a trough where the close, minimize, and maximize buttons are.

[Aditya V (kroq-gar78)]

I think this is doable by adding the trough into the icons themselves, if that would be possible. However, this would raise the issue that the button is highlighted before the cursor is over what the user sees as the button.

[Christopher Kyle Horton (christhehorton)]

Looking in what Ambiance does, the trough images appear to be separate from the button images themselves for the Metacity portion. I'll need to investigate how this layered drawing works, but I think that would be preferable to maintaining buttons with the trough images built-in.

A similar bug for troughs with the Light themes and Unity menu bar already exists (I can't find it at the moment), but AFAIK it is still unresolved.