add: also let enter end transactions if no more default postings

To end a transaction, period is required because there might be another
default posting being suggested, and enter usually means "accept the
default". But having to switch back and forth from enter to period
during data entry is a bit unergonomic, requiring too much thought.

Now, in the (common) case where you have entered a balanced transaction
and there are no more default postings to suggest, enter is also allowed
to end the transaction. Experimental; this might be too inconsistent.