This MR improves color palette UX addressing some of the issues described in ux#51 (moved).
It allows for a more flexible palette tiles layout (in one or more rows) with more control over size and border.
Popup panel reflects changes on the fly making it more intuitive to adjust tile sizes.
Palette selection remains the same, but individual options have a preview:
This MR is feature-complete, but as with any UI change it may be controversial, so it's a "draft".