wayland: Refactor session code

(note: I want to upstream the Session class to kwayland-server one day)

At the moment, the session code is far from being extensible. If we decide to add support for libseatd, it will be a challenging task with the current design of session management code. The goal of this refactoring is to fix that.

Another motivation behind this change is to prepare session related code for upstreaming to kwayland-server where it belongs.