QmlDesigner.desigModeWidget: add navigator to toolbar

QmlDesigner.desigModeWidget: add navigator to toolbar

Now we have a navigation history in the DesignMode.
The check (m_textEditor && editor) is there to keep the history clean.

Change-Id: I26bbefe45b51c53379bfecbe1cdc02a64df76012d
Reviewed-on: http://codereview.qt.nokia.com/519
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>