wl_drm: Add event handler for session activation signal.
wl_drm: Add event handler for session activation signal.

Summary:
Compositor need to change the compositing mode to offscreen or render state depend on session activation state.
this patch is for it.

Test Plan:
(1) Run enlightenment on X.
(2) Change another VT.
(3) Run enlightenment on wayland and drm.
(4) Change the VT to enlightenment which is ran (1).
enlightenment on wayland will not render during session deactivation state.

Reviewers: zmike, devilhorns

Reviewed By: devilhorns

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D1816

rvandegrift/e,tasn/enlightenment,FlorentRevest/Enlightenment,tasn/enlightenment,FlorentRevest/Enlightenment,tasn/enlightenment,rvandegrift/e,tizenorg/platform.upstream.enlightenment,tizenorg/platform.upstream.enlightenment,FlorentRevest/Enlightenment,tizenorg/platform.upstream.enlightenment,rvandegrift/e