branch `bestNewLayer` logic to listen for base and overlay

[itspangler]

@garrettdashnelson - while troubleshooting #111, i realized that there is no logic for loading a "best new base layer". it's hard to notice that this doesn't exist unless you are using a historic map as the base map

could we branch the bestNewLayer logic to listen for both base and overlay layers? my suggestion is here: main...implement-bestNewLayer-for-base-layer

[garrettdashnelson]

@itspangler I'm pretty sure we had behavior like this in Atlascope v1, and then for some reason decided to get rid of it in v2, and never have the base layer execute an automatic layer shift. I can't remember why we did that, though ... and this seems to work well, so merge it in!