add layer categories in sidebar

[futuresoup]

It would be good to have a few category headers to organize the layer checkboxes on the sidebar.

That way we can add more layers and group them by a category header. Something like:

Community Data

• EPA Cleanup Plan
• Community vision

Land Data

• Land Use
• Land coverage
• Noxious weeds

History

• Historical river path
• Duwamish Tribal sites