Export SQLite symbols from QtSql

It would be good if the SQLite symbols would be exported so that the extra functionality in SQLite such as busy handler and commit hooks can be used by the developer.

The handle method provided isn't sufficient for Windows as the SQLite driver compiled into Qt isn't exposed so there is nothing that can be done with the handle. Compiling the SQLite into the application as well creates two SQLite instances in the process, which is likely to cause trouble (handle from one instance used in the other instance).