Add color property to notes

Simple cosmetic change for a better diagram customization.

It would be nice to have the ability to choose colors for notes. This could be implemented as an additional property of note. Initially, it could be a hand-picked selection of ~8 colors with (later) ability to add user-defined colors via color-picker. I think that for better UX, picking the color should also be available after right-clicking the note as the first option above edit/delete buttons.

Example: