Most variable types that do not reference a specific Actor in the level cannot have defaults set in Structs

Most variable types that do not reference a specific Actor in the level cannot have defaults set in Structs. While this makes sense for Actor references, I don't see why a Struct couldn't reference a Material, Texture, Static Mesh, Class, etc.

Tested in 4.6.1 binary and Main (Promotable-CL-2434822)

1. Open any project
2. Create a new Structure
3. Add new variables of type StaticMesh, Texture, Material, and Class Actor

Results:
These variable types cannot have defaults set

Expected:
Variables that do not reference a specific Actor in the level can have defaults set in Structs