Make guest sessions externally configurable

[Robert Ancell]

Currently the guest session code is hard coded into LightDM. This means if guest sessions are enable then any session may be accessed as guest and the AppArmor profile applies to all sessions.

It would be nice to have guest sessions defined as a standard session .desktop file stored in /usr/share/lightdm/guest-sessions. In the case of Ubuntu we would have a unity-guest.desktop which would have a wrapper binary and an AppArmor profile.

With this new method:
- You can limit which guest session types are available (e.g. you can only use Unity as a guest and not KDE by default).
- The AppArmor logic is removed from LightDM (which really can't know what a guest session requires)
- You can add/remove guest session support using package management tools.