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).
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