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.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #525 from marius-avram/pink_object_instance_item

Object instance item changes color when assigning material