The ImageDataToUniformGrid filter for composite datasets should only be available in the GUI if there is a vtkImageData in the composite dataset

This is nitpicking but the ImageDataToUniformGrid filter should not be available in the GUI (i.e. grayed out) if the composite dataset doesn't have any vtkImageDatas in it. See the discussion at https://discourse.paraview.org/t/xml-specification-questions-for-a-filter/2641. Steps to reproduce:

1. Create an UnstructuredCellTypes source
2. Create a GroupDatasets filter

The ImageDataToUniformGrid filter is now available in the GUI but shouldn't be since the composite dataset only has an unstructured grid in it. In reality nothing bad happens but nothing good happens either so we can make the GUI a bit cleaner by removing this option in this instance.

Note that if others are ok with the above behavior, I'm fine with it as well. I don't think of this as a serious flaw in the GUI.