Add auto-generated legend

[nelsonmau]

An useful feature to better understand data in the map it would be an automatic legend sticked on the map.
It could be:

• based on binning (if we have 4 bins, the legend will have same intervals...). Once the bins are changing, the legend will change too.
• switchable by a On/Off button
• arrangeble in one of the four drawing corners (top-left, top-right, etc...)