Expose session functions and session details for current backend

[Timidger]

We should, at the very least, re-export the unsafe functions for the backend (i.e. the ones that grab the VT and thus are fundamentally unsafe).

The only functions that are not unsafe is wlr_session_change_vt and wlr_session_find_gpus. Note there are also getters to wrap. See backend/session.h for more info.