[Bf-funboard] Remove doubles issue & use for UV

[Julio Iglesias]

> Interested to know why it only worked `Good enough`? - Did it work
> with the example image you posted, how should it work better?

I hope I didn't offend you! It is just that using unselected as target
leaves little control in some situations where you have two nearby
loops at the same distance and you only want to merge one. But we can
always slide away the other loop as workaround, it doesn't hurt much.

Anyway, what I really miss is an option to merge at center rather than
just at the unselected position.