Add API providing information if a turn is undoable

[on3iro]

Currently it is not possible to retrieve information about if a certain move is/will be undoable. This could provide useful in situations where one would like to only show an undo button, if the last turn is indeed undoable or if we would like to inform the player beforehand that a move will not be undoable.
The only option to do this now would be to track additional state in G manually.

see #1047 for reference