qtextstream should support setting device's active channel

some time ago, qiodevice gained multi-channel functionality.
it would make sense to allow qtextstream to exploit this conveniently by setting the channel along with the qiodevice.

this is already possible by calling setCurrent*Channel() on the device each time before accessing the textstream, but this is obviously neither convenient nor elegant.

the prime use case would be two text streams on stdout and stderr of a qprocess.

it's conceivable to add this to other serialization classes like qdatastream as well, but i can't really think of a realistic use case for that.