Fix current-data editor for color map editor.

In color map editor, users cannot change the data value for first and last point
using the input-widget for the current item. Instead they should rescale the
LUT. To that effect, we disable the input-widget if the current item is the
first or last control point.

Change-Id: Iec1150a4e19921af10a06dcbafa155cef4f0748b
Merge topic 'fix_edit_control_points' into next

80af5ec Fix current-data editor for color map editor.