Sessions are independent from projects, and this allows for removal of
e.g. the dependency from Bookmarks to ProjectExplorer.
Includes moving the command line arguments (-lastsession, <session>) to
Core plugin.
Change-Id: I6c578fd15c4990902e7196501de20f39376e90e8
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
