Reorder a playlist

It adds the possibility to reorder playlists items via drag and drop. However, when a song is played during reorder, we need to update player's playlist. The only way I found to update player is to call set_playlist method. But this function stops player. I have added a parameter to prevent the stop as a quick and dirty hack. Suggestions welcome.

Closes: #108 (closed)

Update (2 february)

• fix typos, some punctuation and hanging indents
• rename drag and drop functions in playlistview
• playlistview: Correctly use new method _is_current_playlist in _on_playlist_update in
• player: Change set_playlist. Do not stop music and force next_track_validation
• playerview: Replace iter by iter_ and widget by widget_ (python keywords)
• add a docstring to reorder_playlist