Cannot add set/map variables to user defined structs

For 4.15, we should (ideally) support this, or only provide the array option in sturct (if the other path is too big a change for 4.15 post-branch).

1. In the editor, create a new struct asset
2. In the struct, add a new float variable
3. Click on the variable's type icon, and select "Set" from the dropdown

RESULT: Nothing happens (the variable remains a single variable, not a set).