Slider/Text UC: add "hue spectrum" style

Useful for any color-spectrum slider, e.g. Adjustments > Color >
Colorize.  (Note that the Colorize dialog is also updated with this
commit, as it was my test-case).

All that's left to implement for the existing UC styles is a "custom
owner drawn" style, which I created in case I ever need a completely
unique background for a given tool.  But as I don't have a use case yet,
I'm not going to implement code for that just yet.  Instead, I'm going
to start converting various program sliders over to new render modes,
and doing some stress testing to make sure the new UC is solid.
Slider/Text UC: improve "notch" behavior for gradient styles

The same rules apply as standard sliders; display a notch at position 0
if position 0 is not the left-most position on the slider.
Various dialogs: update to use new Slider/Text style options

I *think* this covers all the obvious candidates for non-standard slider
tracks.

It makes me very happy to see how the new UC improves usability of
color-centric dialogs.  For example, the purpose of Split Toning's
"Balance" slider is now abundantly obvious, even to a new user, while
the "Photo Filters" Strength slider now gives a great indication of how
strong an applied filter will be.

As this commit required me to implement a number of different "modify at
run-time" interactions with the updated UC, and everything seems to be
working well, I think the new slider is acceptably stable.  Nightly
builds will resume as normal tonight.