Handle invisible transactions when navigating

[kevinjfoley]

This allows next and prev navigate commands to work better with ledger-occur.

Adds an optional argument to ledger-navigate-start-xact-or-directive-p which indicates whether to consider invisible points when determining if point is at beginning of xact or directive.

This also refactors ledger-navigate-prev-xact-or-directive to use the same recursive logic as ledger-navigate-next-xact-or-directive. I'm not sure if one or the other performs better but I think the recursive logic is easier to understand and it seems reasonable they should both use the same logic.