Better support for fluctuating exchange rates

[mtotschnig]

Currently exchange rate is set on the account level, and each transaction can set an equivalent amount/exchange rate that overrides the account level.

Problem 1) When a new transaction is created, by default the exchange rate from the account level is used, instead from the last transaction or fetched from exchange rate provider.
Thus historical evolution of exchange rates is not correctly reflected unless you each time call "Equivalent amount" from the menu, and download new exchange rate. Even if you do not forget the first, you might just accept the account rate prefilled in.

Problem 2) In the Grand total account, the sum of equivalent amounts is also used to calculate current balance, which gives wrong result

Problem 3) Change of home currency leads to loss of exchange rates.

Problem 3 seems hardest to fix, Problem 2 easiest: We calculate the grand total current balance by applying the most current exchange rate to the current balance of each account.

Suggested solution for problem 1:

Accounts can be configured with static or dynamic exchange rate. With the first option we no longer offer the equivalent amount field to be shown. With the second option, every transaction needs to set an equivalent amount, you will have 2 options either fill it from last transaction for the same currency, or download from exchange rate provider.