The value descriptions of GimpGradientColor,
GimpGradientSegmentColor, and GimpGradientSegmentType enums appear
in the on-canvas gradient editor UI, as combo-box items in the tool
GUI overlay.  Since we want to keep the overlay as small as
possible, we previously used abbreviations for these descriptions
(e.g., "FG (t)", instead of "Foreground (transparent)").

Replace the abbreviated descriptions with unabbreviated ones, and
move the abbreviations to the "abbrev" parameter.  This way we get
the abbreviated version in the combo-box, and the full version in
the combo-box's menu.