title: prevent/force execution of Gtk3 accelerators
body: <p>I've recently decided to write my first Gtk3 app, and I'm having trouble with accelerators. I know how to add accelerators, but they just refuse to work with certain keys, like rightarrow and leftarrow. This is a simple program which should (but doesn't) destroy the GtkWindow when rightarrow is pressed. It works with most other buttons, though.</p>
  
  <pre><code>from gi.repository import Gtk, Gdk
  keyval= 65363
  
  w=Gtk.Window()
  menubar= Gtk.MenuBar()
  filemenu= Gtk.Menu()
  accels= Gtk.AccelGroup()
  w.add_accel_group(accels)
  file= Gtk.MenuItem.new_with_mnemonic('_File')
  new= Gtk.ImageMenuItem.new_from_stock(Gtk.STOCK_NEW, None)
  new.add_accelerator('activate', accels, keyval, Gdk.ModifierType(0), Gtk.AccelFlags(1))
  file.set_submenu(filemenu)
  filemenu.append(new)
  new.connect('activate', lambda a: w.destroy())
  menubar.append(file)
  w.add(menubar)
  w.connect('destroy', Gtk.main_quit)
  w.show_all()
  Gtk.main()
  </code></pre>
  
  <p>Then, on the other hand, there's another problem: if a widget has the keyboard focus (e.g. a GtkEntry) and an accelerator key combination is pressed, the accelerator action is executed - but the widget doesn't receive the key press. Is there a way to prevent this other than removing the AccelGroup from the window whenever a widget is focused?</p>