Add entity to correct managing GstElement pointers.
Use GstElementPtr for playback in CommonPlayer.
Return status from play control methods in player.

Return GstStateChangeReturn from:
 * Play();
 * Pause();
 * Stop();
 * NextTrack();
 * PreviousTrack().
Make test on playing more meaningful.

Check what return Play() method.
Add smart pointer for managing GstMessage pointer.
Make none virtual dtor for GstElementPtr.
Add operator bool for GstElementPtr.
Add check on nullptr during clean up.
Add Bus in CommonPlayer.

Also add method for waiting messages on bus.
Add check in test that after Play track was played.
Add ingnore files for projectes.