Update split parts based on original amounts

[mtotschnig]

Follow up to #962: User reports that they would like to manage split transactions with original amounts in 2 steps:

When the transaction happens, the real amount debited is often not known, so it should be possible to create split parts where only the original amounts are filled in. Currently this is not possible, because for split parts we only allow original amounts if we already know the exchange rate for the parent.
Later, when the real amount is edited, the real amounts for the split parts could be calculated automatically based on the exchange rate of the parent, and the original amounts of the parts.

[gefloh]

I find these foreign currency credit card transactions in general a little tricky because on the one hand I want to put all expenses immediately into the app in order to not forget them later, but on the other hand when I pay in a foreign currency, I don't know the exchange rate until the actual amount is booked on the credit card.