* BFatalErrorException: Add commitTransactionResult property and
respective constructor. In case committing the transaction failed,
BPackageManager throws a BFatalErrorException with the result.
* BFatalErrorException::UserInteractionHandler: Pass
BCommitTransactionResult to ProgressTransactionCommitted().
* Move RepositoryBuilder class to libpackage and add B* prefix to name.
* Pull BPackageManager class out of PackageManager and move to
libpackage. The base class is customizable via three handler objects
responsible for transaction handling, request execution, respectively
user interaction.
* Reorganize _ApplyPackageChanges(): Now we first prepare the
transactions for all affected installation locations (downloading
files etc.) and then commit them.
