PlaylistController: fix inability to play just loaded tracks

...that was caused by asynchronism of playlists and MetaProxy::Tracks.
Now that we have TrackLoader which will magically solve all our
problems. :-)

This fixes Amarok not playing tracks passed to it via command line.

BUGFIXES:
* Fix `amarok --play file.mp3` option didn't actually start playback.

@Anmol, now you can fix the "core" part of the bug 317385 as I outlined
in http://git.reviewboard.kde.org/r/110362/ (please reopen the review).

@Ralf, this is the "recommended" way to handle asynchronism in tracks
and playlists now.

CCMAIL: Chris

DIGEST: Amarok fixes playing of tracks passed on command line