xdg-shell: configure reply race for set_maximized/set_fullscreen

The set_maximized request specifies:

After requesting that the surface should be maximized, the compositor will respond by emitting a configure event. Whether this configure actually sets the window maximized is subject to compositor policies.

However, a client cannot reliably use the configure reply to figure out whether the compositor has accepted or declined the request. Consider the following:

• Client sends set_maximized.
• Compositor sends an unrelated configure event which doesn't contain the maximized state.
• Client receives the configure event, and assumes the compositor has denied the set_maximized request.
• Compositor receives and accepts the set_maximized request, sends a new configure event in reply to the set_maximized request, but it's too late already.

This isn't just theoretical, I've seen it happen in practice.

Thus, forcing compositors to reply to a set_maximized request is misleading and fragile, and should probably be removed.