agl-demo-platform: add launcher and make it work with new App FW

agl-demo-platform: add launcher and make it work with new App FW

Through its dependencies, `launcher` pulls all necessary software,
including `agl-session`, `homescreen`, `weston-ini-conf`... Make the
`agl-demo-platform` packagegroup depend on it for using the new App FW.

Moreover, make the `agl-graphical-weston` packagegroup depend on
`agl-compositor-init` so we directly install the appfw-provided config
and systemd services for `agl-compositor`.

Bug-AGL: SPEC-4161 SPEC-4159
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Change-Id: Ifb84b021925e50a7ea977c25eb711ddf0c9b77bd