QmlDesigner.PropertyEditor: Using envirmoment variable to debug qml

If someone is customizing parts of the property editor then it is
useful to see any warnings from their QML components.
Debugging QML components should not require to rebuild Qt Creator.

The debug enum was not used anywhere else.

Change-Id: I623234312238906d4c4c90e87a5f2bc488f6fc57
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>