Implement universal leaper

[Omshinwa]

[Enhancement request]

Hello,

As far as I can see, right now it's not possible to make a Duck type of piece as an actual piece?
ie: a piece that can move to any empty square and cannot be taken.

Would it be possible to add features so it's possible to create such a piece? (like adding an untakable flag for custom pieces or something).

Thanks!

[ianfab]

Since I usually try to decompose rules as much as possible, I renamed this feature request to specifically only refer to the universal leaper part as it is called in Betza notation (https://www.gnu.org/software/xboard/Betza.html). The feature request for uncapturable pieces is #606.