Add util function to wait for initialization of newly added windows

To detect newly added windows we use the MetaWorkspace "window-added" and the MetaDisplay "window-created" signals. Those signals are emitted very early (the window is not allocated, some properties aren't set, on "window-added" the window actor isn't available) and often multiple times for the same window, also they behave different in Wayland and Xorg sessions. This makes using them quite complicated and introduces subtle bugs which are hard to track down.

Add a waitForWindow() function to make listening to those signals more convenient and use this function for the window-added events used in the overview.

Some of the issues worked around with this: mutter#156, mutter#16, mutter#157