xdg_shell: Adding a new shell protocol.

xdg_shell is a protocol aimed to substitute wl_shell in the long term,
but will not be part of the wayland core protocol. It starts as a
non-stable API, aimed to be used as a development place at first, and
once features are defined as required by several desktop shells, we can
finally make it stable.

It provides mainly two new interfaces: xdg_surface and xdg_popup.

The xdg_surface interface implements a desktop-style window, that can be
moved, resized, maximized, etc. It provides a request for creating
child/parent relationship, called xdg_surface.set_transient_for.

The xdg_popup interface implements a desktop-style popup/menu. A
xdg_popup is always transient for another surface, and also has implicit
grab.