Add ability to clear conqueso values

[jeffreymyers]

The field validation on the conqueso value editor prevents a value from being cleared easily. In the case of text fields, we're having to enter a space as the value to get past the form validation. In the case of a map, property we need to enter something like "dummy=value" to get past form validation.

I think the Edit Property dialog should include a "Clear value" button that will clear and close the dialog without dealing with the field validation.