Switch to a factory-based model for accounts handling

[maccaf]

The current model doesn't adapt well to accounts with special needs regarding transaction, expenses, etc. handlings. Switch to a factory-based model would allow third-parties to implement all special handlings they need.

Things to do:

• Extract an interface to handle account and transactions.
• Add an interface for factories.
• Provide an extension point to hook factory implementations.
• Handle account creation, if allowed, through a factory-specific wizard.
• Handle transaction handling through factory-specific wizards and properties pages.
• Remove the local storage of accounts and trasactions, the factory implementation will be responsible to store the local data, if needed.