Add support to query changeability of playback rate

Scenario:
When user want to fastward or rewind the video playback

Class Member:
void setPlaybackRate(qreal rate)

Reason:
Although void QMediaPlayer::setPlaybackRate() is provided to allow user change play back rate, but it may have no effect because Not all playback services support change of the playback rate. So it would be better if
A. add a function bool QMediaPlayer::playbackRateChangeable() to allow user to query if changing play back rate is supported..
OR
B. bool QMediaPlayer::setPlaybackRate() use bool return value to indicate if the operation is successful.