styling labels

[drzraf]

Is your feature request related to a problem? Please describe.
Some layers' labels are very important. Some other aren't. I'd like to tweak color, font-size, background-color and background-opacity for the labels belonging to one specific layer

Describe the solution you'd like
UI ? Or a textarea to input free-form CSS.

Additional context
Currently, all labels are leaflet tooltips, as a flat list of <div> with no information about the layer they belong to. Having a data-layer attribute would be nice for per-layer label styling.

[yohanboniface]

Closing in favor of #1884