[GTK3] Fix displaying column popup menu

Right-clicking on column header resulted in this error:

    TypeError: could not convert type EventButton to GdkEvent required for parameter 0

The following fixes and cleans up the issue:
- Move the signal creation to the class, using the __gsignals__ dict.
- Replace `Event` with `object` since we are passing an EventButton as
Gtk3 no longer accepts it as an Event.
- Replace deprecated menu `popup()` with `popup_at_pointer()` which also
fixes a critical gdk error when using `popup()`.

deluge/ui/gtk3/listview.py

@@ -12,18 +12,11 @@ from __future__ import unicode_literals
 import logging
 
 from gi.repository import GObject, Gtk
-from gi.repository.Gdk import Event  # pylint: disable=ungrouped-imports
-from gi.repository.GObject import TYPE_NONE, SignalFlags, signal_new
 
-from deluge.common import decode_bytes
+from deluge.common import PY2, decode_bytes
 
 from .common import load_pickled_state_file, save_pickled_state_file
 
-# FIXME: ?
-signal_new(
-    'button-press-event', Gtk.TreeViewColumn, SignalFlags.RUN_LAST, TYPE_NONE, (Event,)
-)
-
 log = logging.getLogger(__name__)
 
 
@@ -87,6 +80,12 @@ class ListView(object):
             Most of the code of this class comes from Quod Libet (http://www.sacredchao.net/quodlibet)
         """
 
+        __gsignals__ = {
+            'button-press-event'
+            if not PY2
+            else b'button-press-event': (GObject.SIGNAL_RUN_LAST, None, (object,))
+        }
+
         def __init__(self, title=None, cell_renderer=None, **args):
             """ Constructor, see Gtk.TreeViewColumn """
             Gtk.TreeViewColumn.__init__(self, title, cell_renderer, **args)
@@ -356,7 +355,7 @@ class ListView(object):
 
     def on_treeview_header_right_clicked(self, column, event):
         if event.button == 3:
-            self.menu.popup(None, None, None, None, event.button, event.get_time())
+            self.menu.popup_at_pointer()
 
     def register_checklist_menu(self, menu):
         """Register a checklist menu with the listview.  It will automatically