Move session management to SessionManagement class
Right now Workspace and SessionManagement were entangled with SessionManager being a dumb class that just signalled to Workspace to do the work. The code, however, lived in sm.cpp despite being part of workspace. This commit is an initial cleanup to strip some X11 specific features from Workspace and move towards having the right code in the right files. It should hopefully help get us in the right direction we get some wayland session management. Behaviour should be unchanged.