kmscon: add wayland-session support

kmscon: add wayland-session support

This adds a wayland compositor which registers sessions for each top-level
window within kmscon. This means, you can run any arbitrary
wayland-application on kmscon. Each top-level window that is registered by
the applications is put into a separate session and you can switch between
the sessions with the previously introduced session-shortcuts.

The compositor is still very basic. It doesn't support input devices at
all (not even talking about DnD). The only thing it does is surface
management (and even that is not fully implemented).

However, this shows the direction that kmscon will go. You can run the
Wayland "simple-shm" and "simple-egl" demos on kmscon to see how it works.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>