Summary:
The WaylandServer creates the XdgShellV5 interface and hooks it up
to create a ShellSurface whenever an xdg surface or xdg popup is created.

ShellClient gains some new ctors for the different variants and is
adjusted to delegate to xdg surface respectively.

With this change KWin mostly supports xdg-shell protocol. Still missing
is support for the "geometry" request which is rather difficult to
implement in KWin.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2108