Add support for gradients of colors

[AymenStudios]

Allow support for gradients of colors in

• [x] shapes (since 0.5.x-unstable)
• pens:
  • [ ] pencil
  • [x] line (since 0.4.x)
  • [ ] curve
• [ ] fonts, text

This property would be great!

[maoschanz]

i edited your comment to have a nice todo list, but notice that linear gradients available from the cairo librairy (which i use here) are not that great, for example a curve with a blue-to-green gradient could be