Group layers so they can be swithced on/off together

[apanagio]

Is your feature request related to a problem? Please describe.
I'd like to have two or more layers with a single legend, so that they can be shown / hidden together. Currently this cannot be done.

Describe the solution you'd like
A solution could be to show one legend for layers that have the same label.

• Legend styling should be taken from the first layer
• In order to not brake backwards compatibility the feature should be turned on with a config flag

Describe alternatives you've considered
I cosidered this: https://github.com/developmentseed/mapbox-gl-layers, but it doesn't seem to work

Additional context
A common pattern when you want to make a line layer easily clickable, is to add an additional layer with opacity 0.01 and much bigger line-width. You don't want an extra legend for that layer and you'd want to toggle the two layers together