Use a typedef for monetary values

Replaces int64_t with a typedef in all monetary contexts.

See #4067 for a more thorough description of this patch and associated context.
BTW we have many signal/slot definitions like this:
```c++
connect(model, SIGNAL(balanceChanged(qint64, qint64, qint64, qint64, qint64, qint64)), this, SLOT(setBalance(qint64, qint64, qint64, qint64, qint64, qint64)));
```
I'm fairly sure that these need to be updated to use CAmount as well. No need to add & or const in these SIGNAL() and SLOT() definitions, but the type needs to match.

Also the CAmount type likely has to be registered to Qt at some point to be able to pass it in this way (see http://qt-project.org/doc/qt-4.8/custom-types.html).

As long as CAmount is the same type as qint64 I imagine it will keep working, but it seems brittle.