In long term, we want to split XdgShellClient into several classes. One
class for xdg-toplevel clients, and the other one for xdg-popup clients.

xdg-popup clients are much simpler than xdg-toplevel clients, they can't
be maximized or shown in full screen mode, they can't be interactively
moved on the screen, and so on. In the end, we will have to plumb many
pure virtual methods, which looks a bit ugly.

This change makes support for full screen mode in AbstractClient optional
so we don't have to add those no-op methods and keep code more or less
"clean."

Compiles.