Add null session

Created by: ascent12

This is just a session backend which does nothing. It's so when we're using a wayland/xorg/whatever (anything that isn't DRM, really) backend, the user can be given this session and not worry about checking what backend is actually being used.

This fixes a bug where closing one of the examples using the wayland backend would absolutely destroy your session (massive visual glitches, unable to change VT), and forces you to restart your computer. This happened to me when using an example under weston under X.

This PR doesn't add "session/null.c" to the build system. I'll add it when/if the meson PR is accepted.

Instead of doing this sort of patchwork fix, we might want to consider whether we want wlr_session to be part of the public API. Maybe all of the session stuff could be handled in wlr_backend_autocreate, so we don't accidentally try to open a logind/direct session when we're not using DRM.