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.