[GTK] Fix drag and drop files in files_tab

Encoutered an error reordering files by dragging in the files tab:
   TypeError: can't pickle TreePath objects

The issue was get_selected_row  now returns a list of TreePath objects which
cannot be pickled. Also the set_text method only accept unicode text to
pickled bytes cannot be used.

The fix is to convert the TreePaths to strings and use json to encode
the list of strings for set_text.

deluge/ui/gtk3/files_tab.py

@@ -9,10 +9,10 @@
 
 from __future__ import division, unicode_literals
 
+import json
 import logging
 import os.path
 
-import six.moves.cPickle as pickle  # noqa: N813
 from gi.repository import Gio, Gtk
 from gi.repository.Gdk import DragAction, ModifierType, keyval_name
 from gi.repository.GObject import TYPE_UINT64
@@ -813,14 +813,14 @@ class FilesTab(Tab):
 
     def _on_drag_data_get_data(self, treeview, context, selection, target_id, etime):
         paths = self.listview.get_selection().get_selected_rows()[1]
-        selection.set_text(pickle.dumps(paths, protocol=2))
+        selection.set_text(json.dumps([str(path) for path in paths]), -1)
 
     def _on_drag_data_received_data(
         self, treeview, context, x, y, selection, info, etime
     ):
         try:
-            selected = pickle.loads(selection.get_data())
-        except pickle.UnpicklingError:
+            selected = json.loads(selection.get_data())
+        except TypeError:
             log.debug('Invalid selection data: %s', selection.get_data())
             return
         log.debug('selection.data: %s', selected)