backend/session: take wl_event_loop instead of wl_display
wl_display holds a lot more than wlr_session needs: wlr_session only needs to wait for a FD to become readable, but wl_display provides full access to the Wayland client and protocol objects.
Switch to wl_event_loop to better reflect the above.
Ideally we should do the same for all backends as well. Backends should have nothing to do with Wayland protocol related stuff.