window-list: Track window-created/unmanaged
Shell's WindowTracker switched to those signals instead of tracking windows through MetaWorkspace's window-added/removed signals.
As ::window-created is only emitted after a window has been added to its workspace, looking up its app on ::window-added now fails because the window isn't tracked yet.
Address this by switching to the same signals for window tracking (which is simpler anyway).
Fixes #372 (closed)