QmlDesigner.propertyEditor: bugfix for assert

Since the linedit in the property editor has an input mask,
propery names like "red" are not handled correctly.

Change-Id: I43ec2f7e42de7f57d78c97b6865ea36c4e11b3fb
Solution: We normalize "red" to "#ff0000" in the property editor.
Task-number: QTCREATORBUG-5479
Reviewed-on: http://codereview.qt.nokia.com/1749


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