Add gimp_item_get_merged_color_tag(), which returns the color tag
of the nearest ancestor (including the current item) that has a
color tag other than NONE.  Use this function in GimpItemTreeView,
instead of gimp_item_get_color_tag(), to set the cell color of
items, so that item's with a NONE color tag inherit the color of
their parent.  Add a boolean "inherited" parameter to
gimp_get_color_tag_color(), which indicates if the color tag is the
item's actual color tag, or an inherited color tag, and modify the
returned color accordingly, so that inherited colors are less
saturated/lighter than non-inherited ones.