02_agl_compositor.md: How to change default ref UI

02_agl_compositor.md: How to change default ref UI

02_agl_compositor.md: Add simple client examples + how to
incorporate/integrate a different UI.

Bug-AGL: SPEC-4832
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I5750c309dc36f5a87bbeab492efa730e7e0c708a
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/29724
Reviewed-by: Lisandro Perez Meyer <lpmeyer@ics.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>