Add legends for each MBTile layer

[Ben Wyss]

The MBTile layers need legends. One approach could be to create a html legend that is embedded in the MBTile set. The issue with this is that when two or more MBTile layers are rendered on the same map they will both have legends that will be conflicting.

A solution would be to create a legend server (simple html files/snippets). When each layers is rendered in a leaflet map, the legend is also returned to the client. Then the user can add or remove any available legend to a 'legend dialog', this legend dialog can be viewed or hidden.