Gradient support for element color

[gallantcrusader]

It would be cool to have a gradient as color customization. People would be able to make old aesthetic rices.

[FelixKratz]

I think it will be quite a bit or work to properly introduce gradients and possibly other color effects. Those should be available to all elements which receive a color argument. Maybe inspriation can be drawn from the way I implemented it in JankyBorders, i.e. color=gradient(top_left=0xAARRGGBB,bottom_right=0xAARRGGBB) as a special value for a color, but not sure yet.