QmlDesigner.formEditor: disabling double click (enter component)

There is now an item in the context menu.

Change-Id: I5b7cc9ec5c374354e546a650f7c59539ca81ab29
Reviewed-on: http://codereview.qt.nokia.com/1512


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>