Welcome Screen: Fix regressions

- last session / current session semantics is back
- Manage Sessions... button is back

This commit changes the behaviour of currentSession()
to return the session name and not the full path.
SessionNodeImpl as the other user has been adjusted
accordingly.

Change-Id: I1dcfbef2fb4dacf3e3906871d816483c2bfb76da
Reviewed-on: http://codereview.qt.nokia.com/1461


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>