title: How to get a GObject.Callback from PyGObject?
body: <p>I'm trying to add callback functions to a <code>Gtk.Builder</code> using <code>Gtk.Builder.add_callback_symbol</code>. I tried to pass a python function to it, but that does not work. <a href="https://lazka.github.io/pgi-docs/Gtk-3.0/classes/Builder.html#Gtk.Builder.add_callback_symbol" rel="nofollow noreferrer">Documentation</a> says I need to pass a <code>GObject.Callback</code> instead, so I tried to cast one by calling <code>GObject.Callback(myfunc)</code> but got a <code>NotImplementedError</code>. The <a href="https://developer.gnome.org/gobject/stable/gobject-Closures.html#GCallback" rel="nofollow noreferrer">C-Documentation on GCallback</a> says I need to use something called <code>G_CALLBACK</code> to typecast. But there does not seem to be any reference to this in PyGObject and I'm lost at that point.</p>
  
  <p>I would like to say beforehand, that I know callback can be also added by using 'Gtk.Builder.connect_signals', but that's not the question here.</p>