Allow DropDown option to have a default value.

[robertjf]

When creating a drop down with two options, one that shows other properties and the other hides them - I'd like to be able to set a default value so that those properties are hidden by default.

e.g.:

By default, the "Blobs" Option might be chosen, thus hiding the two properties backgroundColor and highlightColor.

Currently the result is this:

When this would be better: