Interface for making multiple edits in the same game tree mode

[dexonsmith]

From the forums:
https://forums.online-go.com/t/condense-edited-position-in-game-tree/50637

Currently OGS displays edited moves in the same way as regular moves. If you edit in multiple stones, each edited stone will show up as an individual move in the game tree. This makes it so if you make large edits to the board position or set up a problem on the board, it shows up as a long string of individual moves. This then makes it extremely confusing to navigate the game tree. Other programs will often condense all of these edits into a single icon on the game tree which greatly helps with tree navigation.

The GoEngine can't handle multiple edits in the same game tree node, so the first step is online-go/goban#152.

Once the GoEngine's model supports multiple edits in the same game tree node, this issue tracks create an interface.

A proposed interface is:

• When "alternating moves" (the OGS logo) is selected, no changes.
• Else, the "Pass" button disappears, and is replaced with a button called "New node" (or similar).
  • "New node" will creates a new node in the game tree (with no edits yet).
  • Same as a lonely ; in the SGF.
  • These edit-only nodes should have an icon that's distinguishable from white/black/pass in the game tree.
• All edits happen on the current move node.
  • E.g., editing in a black stone at coordinates ff adds AB[ff] to the SGF, but does not add a ;.