plugins: add Leaflet.ActiveLayers plugin by vogdb (#5857)
plugins: add Leaflet.ActiveLayers plugin by vogdb (#5857)

Author already submitted another plugin (Leaflet.SelectLayers) himself some time ago (see commit https://github.com/Leaflet/Leaflet/commit/18e0061450a4b1602822ba3f6c7950fd1ef07688)
stefanocudini/Leaflet,msiadak/Leaflet,CapeSepias/Leaflet,Leaflet/Leaflet,msiadak/Leaflet,Leaflet/Leaflet,msiadak/Leaflet,Leaflet/Leaflet,CapeSepias/Leaflet,stefanocudini/Leaflet,CapeSepias/Leaflet,stefanocudini/Leaflet