layout/shell: Add basic support for split window

This introduces a new set_split request to allow changing the tile
orientation of the window. See the protocol XML for more implementation
details.

Of importance difference from the previous implementation is that
this patch makes use of the xdg-shell protocol, such that orientation
is being handled over the configure event to the client.

The protocol specifies a width to allow the client to control how much
of the output be assign the split window and also a sticky window
functionality.

Bug-AGL: SPEC-4839
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia8b7d04a7514f55d647c3ea76b13bab51a3586aa