Improve shell window stacking ordering

[Philip Withnall]

Here’s a series of patches which work towards improving the shell window stacking. They clean up the code and implement a more organised approach to stacking. They add a new test client, weston-stacking, for testing window stacking and ordering.

I don’t claim the ordering they impose is perfect, but it seems to at least be on parity with the current behaviour, and fixes the following bug:
    https://bugs.freedesktop.org/show_bug.cgi?id=69443

Open questions for review:
 • This probably doesn’t work with non-shell_surface surfaces.
 • Do we expect popup (and transient)-type surfaces to be parentable to other popup or transient surfaces? Or is the hierarchy at most one layer deep?