libgimpwidgets: arrange color models in tabs in GimpColorScales.

This allows to have a smaller and cleaner color dock instead of just
listing all possible channels (which may only grow up as we may add more
color spaces).
The API gimp_color_scales_(set|get)_show_hsv() are removed in favor of
more generic gimp_color_selector_(set|get)_model(). I assume this is
ok since they have only been available in the dev version (commit
6258d525, a month ago).