Improve fullscreen handling in Xwayland

This MR supersedes !424 (5fd14185).

The original (adjusted) description follows:

A patchset for improving fullscreen related behavior for xwayland surfaces. See the commit descriptions for more details. Here is an overview:

The first patch enables shells to properly transition an xwayland surface to the fullscreen state. Without this patch, xwayland surfaces are resized to the proper size, but are not moved to cover the whole screen, and the still display window decorations.

The second patch updates xwayland to inform shells when a window loses its fullscreen state.

I'll quote from the comments in the original MR to continue the discussion.