Add a Raise() method

[selurvedu]

Currently I'm emulating this by reading the bus name of the currently active player from Empress and then invoking the Raise() method on the corresponding MPRIS interface. Of course, this method is not available for all players – for instance, it doesn't work for Chromium-based browsers.

[ray-kast]

Done (hopefully)! This should be ready to go once 3.0 is ready for release.