Add legend of selected layers

[lecrabe]

A important feature to be added is the legend of the selected layers (year of degradation, deforestation or annual change map) with the color coding (as it is already provided for the GFC map) in the "Display" screen.

[dfguerrerom]

I think we can add this using the visualization on https://forobs.jrc.ec.europa.eu/TMF/resources/tutorial/gee#tutorial1