Fix scaled UV on import, add widgets

[JamesTKhan]

UV attributes were not being imported into Mundus for GLTF models. This fixes that by getting and setting these values on GLTF import

    public int uvIndex = 0;
    public float offsetU = 0;
    public float offsetV = 0;
    public float scaleU = 1;
    public float scaleV = 1;
    public float rotationUV = 0;

NOTE: A known limitation of gdx-gltf PBR shaders is that it uses UV scale from one of the textures. Separate UV scale per texture type is not supported right now however, this PR adds UV scale props into each material type so that is could be supported later.

Now scaled UV will import into Mundus. Also adds new UI widgets to allow customizing these values.