Add support for more note colors

Seems like for Microsoft whiteboard, people want more note colors:

https://whiteboard.uservoice.com/forums/915919-whiteboard-for-windows-10/suggestions/39266938-rgb-color-for-notes

Could also be an option to change the color of post-its in our diagram.

We could also think about different note styles, like post its, plain and some kind of index cards that have different colored lines.