Feature/gradient color picker@mav

[TheMaverickProgrammer]

Adds the following features:

• optional initial background color
• new gradient picker abstraction with an equation callback to color the BoxDecorator
• comes with a slider widget to indicate where the color was clicked
• gradient picker used to sample colors from the rainbow
• gradient picker used to sample luminosity
• can hold click/drag finger to draw continuously

Changes:

• Not using the color buffer to store palette indices. The color is read from the palette list and drawn to that pixel directly
  • this was needed to let background colors work
• Palette index is now optional in the callback tap details object for this reason. colorValue was added to be more reliably used as you will be guaranteed to have a color
• Color palette is now optional. if color palette is null, the editor defaults to using the rainbow and lumen color pickers

Preview:

I hope this PR is accepted. I want to keep adding features. I was thinking of adding a lasso tool, flood fill, and brush sizes. Let me know if that is interesting to you.