Optimized torrentview.cell_data_trackericon

cell_data_trackericon would load the tracker icon with
gtk.gdk.pixbuf_new_from_file_at_size each time it's requested.
These regular requests acumulating to thousands calls to
pixbuf_new_from_file_at_size with a big torrent list.

Now, read the tracker icon from disk once, and cache it.