Implement gradient for widgets

[ishitatsuyuki]

It seems Gradient is defined in the color module but never used.

I would like to use that for some widgets instead of flat color.

Probably PistonDevelopers/graphics#1076 should be implemented first.

[mitchmindtree]

Just want to add here that conrod does now support gradients, though only via the glium and gfx backends. I don't think there's a way to colour individual vertices different colours in piston yet, so piston backend just shows flat colours for now.

Gradients can be achieved by using the Triangles object with Triangles of ColoredPoints. There is not yet any high-level API for displaying gradients and I haven't yet thought much about how to provide this. Any suggestions or references to other high-level gradient APIs welcome!