add a new playlists window entry when dropping a playlist into it

[ciros88]

current behaviour: dropping a playlist into the playlists window adds all the tracks into the current playlist

desidered behaviour: dropping a playlist into the playlists window should adds the dropped playlist as new playlists entry, without switching from the currently played playlist.

having that, how it should behave bzr2 when the playlist has been dropped in: it shoudl continue playing the current playlist or switch to the new one? this behaviour should depend on #139 checkbox "enqueue items"

[ciros88]

2.0.40.Alpha:

this enhancement has been also implemented when dropping a playlist to tracklist window also. if the user drops a playlist file in the playlists windows it should be added as new playlist entry like is doing now, but if the user drop the playlist in the tracklist window it should behave as before, ie adding that playlist tracks to the current tracklist. an example of this scenario: when an user wants to "sum" (merge) multiple playlists he drops them into the tracklist window; now this is not possible anymore

[ciros88]

in the same manner, when dropping tracks on bzr2, they should be added to the tracklist where the mouse hovers on, but if the mouse hovers on the the playlists windows then a new playlist (with a default progressive number) should be created with all that tracks added into.