feature request: cycle current path through leaves underneath the current node

[petermao]

In order to switch variations, I find myself going past the branching node, using left/right to switch branches/variations and then moving back up. It would be nice to have a keybinding/command that cycles the path through the leaves (ending nodes) under the current node.

This differs from the current "go to previous/next variation" bound the left/right (v0.51.1) in that the current node does not change, only the path underneath it. Also, it may address the underlying desire of #552 (Cycling through direct siblings), though not exactly (as this cycles through child paths).

I would be happy to attempt to implement this if someone points me to the relevant part of the code.

[petermao]

@apetresc, thanks for merging this. Please take a look at @kevinsung's PR's as well. I've been using his dev branch merged into mine for a long time. It's stable and brings in useful features.