Add wayland globals

This PR adds a wayland display singleton.
Targets #74 and #75.
Still WIP!
I think I will exchange this with `wl_display` and make the loop public.
This has two reasons:
 * We will most certainly need to lock the `wl_display` singleton.
 * The loop is a member of `wl_display`, which makes it easily accessible through the functions provided.
 * It will probably make a cleaner interface