QmlJSEditor: only show context menu for Quick Toolbar if available

Task-number: QTCREATORBUG-4521
Change-Id: Iedd97549c6f724b58a0727694ea1b34a2c70d9e9
Reviewed-on: http://codereview.qt.nokia.com/2264


Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>