Adds support in src/core for tracking override redirect windows.
The metacity core is now directly aware of override redirect windows. - They get MetaWindows like normal windows, so mutter no longer need to special case them. The net wm types applicable to override redirects are also recognised. - The original positioning/stacking semantics of metacity with respect to OR windows are maintained, but now it's easier to synchronize the stacking between the core and mutter. - mutter no longer needs to manualy track UnmapNotify, MapNotify, ReparentNotify, CreateNotify and DestroyNotify events; instead map, unmap, add and remove events can now be consistently delivered via the MetaCompositor callbacks.
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