Represent initial state of PlaylistsMediator class.
Move player panel controls to related class.
Insert PlaylistsMediator into PlayerPanel.
Rename PlaylistsMediator -> PlayerMediator.
Introduce PlayerClient, PlayerInterface, ObserverInterface classes.
Introduce BridgeConnector class.

It provide tunel between one Observer and one Player.
In future it should be extended to many Observers.
Introduce Playlist wrapper between Interlayer and interface.
Provide simple methods to PlayerMediator which handle playlist events.

Player is able to create empty playlist and react on adding new tracks to playlist.
Necessary callbacks are passed to client of player.
Pass track into current playlist on its activating in observer.
Remove useless spaces in MediaObserverTreeModel.
Add tree model for playlist table.
Set tree model of playlist to test page.
Provide initial logic of adding track in playlist view.
Pass new created playlist to the interface layer.
Provide Track wrapper and getting its from playlist.
Provide adding one track from observer to playlist.
Provide proxy methods to control player.
Connect playing current track to button.
Connect buttons stop, next, previous track to implementation.
Merge branch 'add-player-mediator'