OSX: Completed Song Browser contextual menu + bug fix in SongGridViewControl.

- ISongBrowserView/SongBrowserPresenter: Added
OnSongBrowserAddToPlaylist action and added SongBrowserError method.
- Fixed/compeleted Song Browser contextual menu.
- SongGridViewControl: Fixed bug where the mouse on header would not be
invalidated when the mouse was leaving the header from below.