Separate QIF export files by transaction currency

up to 913120d , it only deal with export to SD card, and will break export by file sharing ( due to the altered QIF file, see bellow)

Altered the QIF file in the internal storage. Transactions with grouped by currency, and each group will have a "*XXX" line at the beginning, where XXX is the currency code.

Then, when export to SD card, this file is split up into multiples files in `copyFile()`, each file containing only transactions in one currency, according to the currency tags added previous. The `currencyCode` is attached to the exported QIF file name.

Nothing has been done on exporting by file sharing yet. Suggestions are welcome.

This will break the QIF export `MigrationHelper`. I didn't seem to find where the QIF export in `MigrationHelper` is used.