I'd move the event setup into addTo, but JS is utter garbage and thinks
"this" in a function callback like that should be whatever, not where you
actually define it.
Instead of stuffing specialised values into overviewer.collections,
put a reference to the overviewerConfig tileset entry into the leaflet
layer, so that we can access it from callbacks easily.
Many things do not work at the moment:
* Missing world selector
* Missing overlays (overlay tilesets and markers)
* Missing urlhash code
* Probably other stuff
