Add vertex color import options (ignore and so on) for skeletal mesh import

Since 4.16 you are able to paint vertex colors on skeletal meshes in UE4. Right now it’s not super useful though because if you reimport the mesh all of the things you painted in engine will be replaced by the vertex colors in the reimported file. Skeletal meshes should probably have the same options for importing vertex colors as the static mesh import. (but having “replace” be the default and automatically switch to “ignore” for that mesh after using the paint tools on it)