Current implementation was always making 2 rows. Yet sometimes the dock
can be made wide enough to accomodate all the color icons on 1 row,
hence saving vertical space, as well as not leaving this ugly space on
the side of the widget.
Now the widget will be able to compute an optimal size. Therefore if
there is enough space to align all color icons on a single row, this
will be organized this way. Otherwise, the widget will request more
vertical space to use 2 rows, hence allow a narrower dock as well.
Basically this will adapt to how you organize your docks, depending on
whatever screen size, pixel density, etc.
New GimpColorHistory widget, replacing the code in GimpColorDialog, and
added to GimpColorEditor to have the color history accessible in the
color dock as well.
Thanks to Thomas Manni for the initial implementation attempt.