QmlDesigner.formEditor: do not show items from a delegate

Items that have a custom parser or are components are not shown
in the formeditor.

Task-number: QTCREATORBUG-5228
Change-Id: I633fe35d40edd1a5b14275d67a48265313b08657
Reviewed-on: http://codereview.qt.nokia.com/830


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Marco Bubke <marco.bubke@nokia.com>