They're no longer TextureButtons, but Buttons with TextureRect as children. This makes it easier to make them work for multiple themes, and we also save some disk space.
More buttons will follow soon.
- If the palette has colors, automatically select the first one
- When adding a new color button, take the color currently selected on the picker instead of white
- Fixes issues with drag&dropping color buttons. The color that was dragged remains selected.