QMediaPlayer should select its backend depending on the media played

Currently, if several backends are available on a given platform, the first one will always be selected to play a media. It should instead select the one most likely to be able to play the media.

For example on Windows there are two possible backends, DirectShow and Media Foundation, with different codecs support. It would be nice to provide both plugins and let QMediaPlayer choose which one to use depending on the available codecs.