SPEC-3723: restructure meta-agl-demo

All demo related components should be in here now.

We keep the packagegroups on purpose for now to stay backward-compatible.

v2: layer does pass yocto-check-layer, dependencies adapted
v3: remove the dynamic-layer setup, use all-in-one approach
v4: Fixed comments from Paul Barker. Tnx!
v5: Removed wayland/weston/agl-compositor additions, except for demo
    specific weston-init bbappend

Follow-up changes required later:
      - massaging packagegroups
      - scrub of recipes

Bug-AGL: SPEC-3723

Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I47cefd8c23d46b2cdd063470e3f7d97d5ad952d8