shell: Put xwayland surface at the top of the fullscreen layer

shell: Put xwayland surface at the top of the fullscreen layer

An xwayland surface corresponds to a override-redirect window under X,
which is typically a menu or a popup window.  They typically appear
with a keyboard and mouse grab and by nature of being override-redirect
these window can appear anywhere on screen and in the stack.

We need to resort to heuristics to decide where to place the
override-redirect in our surface stack, and for now we'll just put it on
top of everything.  That's going to be correct for almost all cases of
clicking to open a menu, but we can revisit and refine if we run into
a case that needs better handling.