QmlDesigner.PropertyEditor: Using regExp instead of inputMask

This does work as expected.

Task-number: QTCREATORBUG-12018
Change-Id: I1ae4c2f9c222ffcc68a81ab545fe19ac253b0dbb
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>