Add support for wl_shell_surface::set_popup and popup_done

Summary:
This extends the client side API to support creating popup ShellSurface
windows and the server side API to send out the popup_done request.

This is needed to properly support popup windows (e.g. context menus)
in KWin.

Reviewers: #plasma_on_wayland, #frameworks, #kwin

Subscribers: plasma-devel

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D5174