Performance optimization of the mymoney object container.

The MyMoneyObjectContainer::account(QString) was the most costly
method of this class. Mainly due to the calls to dynamic_cast
needed to obtain the cached MyMoneyObject with it's proper type.

Improved this by:
1. implementing a per object type cache.
2. using QHash instead of QMap which provides faster lookups
3. replace macro method implementation with template methods

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