Colorpalette: De-linting.
Colorpalette: Making _makePalette() testable by removing reliance on its being inside a widget.
Colorpalette: CSS: The preview and its container work just fine without display: table
Colorpalette: Ensure base colour is part of the colour palette generated by _makePalette()
Unit tests: Colorpalette: Give the widgets IDs
Unit tests: Colorpalette: Ensure that _makePalette() creates a palette that contains the colour on which it is based