EngineController, Fadeouter: improve fadeout behaviour by using RAII principles

Note that this needs phonon-gstreamer bug 313551 solved to unleash the
full power.

BUGFIXES:
* Fix bumpy fade-out when first used.
* Fix subtle bugs when a lond fade-out is initiated near the song end.
* Don't try to fade-out with phonon backends that don't support it. (vlc)


FIXED-IN: 2.8