non-session-manager support

Implements open, save and dirtiness.
No error checking, no menu items disabling.
Prevent the new session dialog from appearing
Make sure every imported file is copied to the session path
Better start up

Don't reply to nsm server until ardour project is finished loading
Disable some actions that are not allowed while under nsm
Merge branch 'nsm' of https://github.com/royvegard/ardour