Separate render labels and render legend

Now two seperate options
Change group_label to group for consistency
Allow passing a single color
Make the landmark group vs label clearer

Use : instead of _
Update docs for new kwargs
Change render_labels to render_numbering

Update widget to allow you to toggle it on and off
Fix bugs introduced by show_numbering

Seems to work for all widgets now
Merge pull request #486 from patricksnape/image_vis_improvements

Image visualisation improvements