grid-cell: implement star emblems

Nautilus does not display anything if the user selects "Star" as caption for for the grid view.

This is because the star caption quark is not handled by nautilus_file_get_string_attribute_q, which causes us to set NULL for the caption label.

As a more definitive solution, implement the display of star emblems for the grid view instead of relying on captions. Also remove the option to select star captions.

Fixes #2842 (closed)