Fix seeking to the end of a track.

Issue highlighted here: https://bugs.tomahawk-player.org/browse/TWK-1703

By doing this I had to disable the scrubbing behaviour (Which wasn't quite there anyway. I'd think you'd have to perform this by altering the playback speed, and only seeking for instantaneous changes. Obviously this wouldn't work well for remote sources though.)

The seeking occurs on mouse click, not on mouse release. idk if this is preferred with you guys?
Always prefix member variables with `m_`