We currently upload the runtimes and demo apps to different flatpak
repositories, which is not common practice and imposes an additional
configuration step when installing a flatpak from Apertis.

Switching to a single repo holding both the runtimes and apps eases
things a bit and aligns us on what others do.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>