Better heuristics for default color in annotation toolbar

[RFSH]

Currently, the toolbar is always showing red color by default. Since users wouldn't want to reuse colors for different annotations, this means that they almost always have to pick a color before drawing. We should improve this. The following are the improvements that we can apply:

• In edit mode, we can change the color to be the main color that is used in the existing annotations.
• In create mode,
  • Simple solution: Randomize the default color. Each time a user clicks on "create" we can generate a random color to ensure "uniqueness".
  • Better solution: Have a pool of colors and randomly pick one that is not used by other annotations. This way we can help users with picking a color blind friendly palette.

[RFSH]

I ended up doing the described "better solution". So the code will now pick the first unused color from the color palette described here or a random color if all the colors are used.