WPrefs: set default to 1st color in gradient texture
This patch is updating the gradient texture user experience
by auto selecting the first color in the list when possible
and fixing the autoselection after a delete action.
In fact what that is called gradient colors is just a list of colors
with hue/saturation/brightness value.
That patch is loading the first one from the list (color, hue,
saturation, brightness) and setting the 3 sliders according to that
value instead of using some default hardcoded values.
The default color value on the top-right is never modified.
Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>