QSoundEffect: Don't try guess a media role if one has not been provided.

Trying to heuristically guess this kind of thing is almost always a bad idea.
Expect the creator of the output to explicitly tell us the category instead
of guessing.

Change-Id: I5f2988e2456685f3622e0ab136951b1742215f71
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
QAudioOutput: Don't guess media role if one has not been provided in setCategory.

Trying to heuristically guess this kind of thing is almost always a bad idea.
Expect the creator of the output to explicitly tell us the category instead
of guessing.

This fixes QAudioOutput not respecting system volume on Sailfish.

Change-Id: If1d05192f513eb54fdfbd1df217286f329b2bfe8
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>