[LMP] Initial migration on pure gstreamer.

Plays something.
[LMP] Use URIDecodeBin for streams support.
[LMP] Fixed a typo.
[LMP] AudioSource::Clear().
[LMP] Removed some old debugging output.
[LMP] Migrate to playbin2.
[LMP] Register AudioSource metatype.
[LMP] Somewhat working gapless playback.
[LMP] Implement SourceObject::Get{Current,Total,Remaining}Time().
[LMP] SourceObject now generates ticks.
[LMP] Implemented IsSeekable().
[LMP] Pass nullptr instead of 0.
[LMP] Style fixes.
[LMP] Return true from the callbacks.
[LMP] Support seeking.
[LMP] Emit stateChanged() after updating state.
[LMP] Volume control.
[LMP] Debug about unknown message types.
Merge branch 'lmp_gstreamer'