The currency editor dialog now properly handles edit triggers.

The currency ID column is not editable as is should have been.
Moved the whole currency updating business logic to KMyMoneyApp
where we already had the currency creating, renaming and deleting
methods. Moved the initial loading of the currencies out of the
dialog constructor so that the initial currency selection will
enable the proper actions.

BUG: 324172
REVIEW: 112350