QmlJS: Set correct scope in signal handlers.

This means the code model will now offer correct completion and
highlighting for arguments of signals in their handlers, example:

MouseArea {
  onClicked: {
    mou<complete> // now also completes 'mouse'
  }
}

Reviewed-by: Fawzi Mohamed
Change-Id: I01838ef00e391b13e6e5a832c9ec3cd983689c5b
Reviewed-on: http://codereview.qt-project.org/6147


Reviewed-by: Christian Kamm <christian.d.kamm@nokia.com>
Sanity-Review: Christian Kamm <christian.d.kamm@nokia.com>