The register now permits a transaction to have splits with different
commodities, but doesn't do the right thing to transfer the commodity over
when adding the split to other account. The transfer dialog does do the
right thing.

Note that currency accounts are no longer necessary to achieve the
transfer, but there is obviously some extra step required that is not
happening from the register, but is happening from the transfer dialog.

I have marked this major because the register just silently accepts the
transaction, but does not deal correctly. We either need to add the extra
step or re-enable (as a temporary measure only, I hope!) the check to stop
transactions from the register having more than one commodity.
This also applies to transfers to stock accounts - they are accepted but do
not do the transfer properly.

I suspect both cases are related to the use of prices (and the lack thereof
from the register).