PATCH: Exceptions thrown when queuing tracks

[Dustin Spicuzza]

This is a bit of a grab bag of fixes, but I'll post the stack traces that no longer happen because of this fix:

User Action: On double clicking on the side panel when the queue is highlighted:
    Traceback (most recent call last):
      File "/home/virtuald/src/exaile/exaile/xlgui/main.py", line 300, in <lambda>
        self.on_append_items(items, sort=sort))
      File "/home/virtuald/src/exaile/exaile/xlgui/main.py", line 471, in on_append_items
        offset = len(pl.playlist)
    AttributeError: 'QueuePage' object has no attribute 'playlist'

    Then when you fix that...

    Traceback (most recent call last):
        Traceback (most recent call last):
          File "/home/virtuald/src/exaile/exaile/xlgui/widgets/playlist.py", line 929, in update_icon
            self.set(iter, 1, self.icon_for_row(position))
        TypeError: iter must be a GtkTreeIter

User Action: Drag/drop from playlists panel to playlist:
    Traceback (most recent call last):
      File "/home/virtuald/src/exaile/exaile/xlgui/main.py", line 300, in <lambda>
        self.on_append_items(items, sort=sort))
      File "/home/virtuald/src/exaile/exaile/xlgui/main.py", line 471, in on_append_items
        offset = len(pl.playlist)
    AttributeError: 'QueuePage' object has no attribute 'playlist'

User Action: Hit play when queue page is showing:
    Traceback (most recent call last):
      File "/home/virtuald/src/exaile/exaile/xlgui/main.py", line 798, in on_playpause_button_clicked
        player.QUEUE.set_current_playlist(pl.playlist)
    AttributeError: 'QueuePage' object has no attribute 'playlist'

User Action: Enqueue all tracks in a playlist (right click, left panel):
    Traceback (most recent call last):
      File "/home/virtuald/src/exaile/exaile/xlgui/panel/playlists.py", line 608, in <lambda>
        self.emit('queue-items', self.tree.get_selected_tracks()))
      File "/home/virtuald/src/exaile/exaile/xlgui/panel/playlists.py", line 1192, in get_selected_tracks
        return playlist.get_tracks()
    AttributeError: 'Playlist' object has no attribute 'get_tracks'

These fixes have been pushed to my dj_improvements branch.