Improve usability of exchange rates

[mtotschnig]

User pointed out 2 important shortcomings of the current state of exchange rates:

1. It should be possible to define global exchange rate, in addition to the current account specific one:

It would be much better to have currency settings as a separate menu, where you could set up all currency rates in one place instead of having to change currency rates in each account's individual settings. That way it would take much less time to update exchange rates when rates change significantly or when base currency is changed. When you have many accounts this is a must. ... And you might very likely forget to update currency in some accounts, which will lead to wrong totals

2. Currently it is not easy to keep up with frequently changing exchange rate, because there is one rate for the account, and when a new rate is used for a current transaction, future transactions will still use the old rate by default. User suggests the following approach:

There is one general exchange rate and an option to update individual transaction rates. So the account exchange rates doesn't change. I would suggest to update the account exchange rates everytime when you make a transaction in that currency. That way your rates history will reflect real dynamics. So all transactions will have historical valid rates. For example: you set USD to SAR rate as 1:3.75 in the settings. Then you make a transaction and change the rate or download it automatically as 1:3.72. So this rate will be saved for this transaction and it will be recorded in the currency settings as well for all future transactions until changed.