Several minor UX issues with Color Picker dialog

[michaelgundlach]

Description
The Color Picker dialog has a few papercuts that could probably all be addressed in one patch. Let me know if I should file separate bugs for all of these.

1. "Click to switch between primary and secondary color" button has blank icon, should be the double arrow icon.
2. Primary and secondary color selectors have no tooltip, should say "Set primary color" and "Set secondary color"
3. "Show Value" checkbox seems to have no effect. If it does something, it needs a tooltip to explain its effect.
4. The title "Color Picker" is already used by the Color Picker tool. Retitle to "Colors" to match Paint.NET.
5. "Reset Color" button text should change to "Reset", since it resets two colors, and saying "Reset Colors" next to the new "Colors" window title would look odd; and so that it matches the Layers window's "Reset" button.
6. Pressing escape does not press the Cancel button, like it does the Layers dialog. It does nothing.
7. Pressing enter does not press the OK button, like it does the Layers dialog. It clicks the "swap colors" button.

Version
Mac Ventura 13.7, Apple M1, Pinta 3.1 beta

[cameronwhite]

I'm ok with having these all together, although the missing icon is a separate issue - I've noted it in #1796 since it seems to be a related issue with certain SVG icons