DisneyMaterialCustomUI: saving values in attribute editor

When the material was opened with the attribute editor and the
parameters where changed they were not saved. Now they are.

This required ICustomEntityUI to be a QWidget with a signal.
Since Qt doesn't support metadata in nested class the ICustomEntityUI
was moved to a separate file.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>