`extendLayerControl` should be `extendLayersControl` + some modifications

[tim-salabim]

@trafficonese I renamed extendLayerControl() to extendLayersControl() (mind the 's') to be in line with the original leaflet::addLayersControl().
I also adjusted a few things regarding CSS and default icons. Among other things, I added package {fontawesome} to Suggests, because I like those icons much better than the emojis.

All in all it now looks like this:

I am still not sure whether this is the end of what I have in mind, but I wanted to let you in on these changes as early as possible, given that you did all the work on this so far.

I hope that is OK with you.
18b80d6

[trafficonese]

looks great to me :)

[tim-salabim]

There's still a few things I nee to iron out, but I am quite happy with this myself! Thanks again for all the great work!!