Object instance item changes color when assigning material

Until now the object instance item stayed pink when a material
was assigned. Now this issues has been solved and the style
is updated when a material is assigned through the
MaterialAssignmentEditorWindow.

Issue #509

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Move to public section at the top of the class definition (we try to keep a single section per visibility level).