Garbage Collection frees needed objects

[mjakeman]

We have several instances where delegates and callbacks get garbage collected when the computer is low on RAM. A typical error message looks something like this:

Process terminated. A callback was made on a garbage collected delegate of type 'GObject!GObject.WeakNotify::Invoke'.
   at Gtk.Global.main()
   at Gtk.Global.main()
   at Gtk.Global.Main()
   at GtkDemo.Program.Main(System.String[])

Once #52 is merged, I will try look into this along with some other changes to lifecycle code (e.g. ToggleRefs), so not very pressing at the moment.

[mjakeman]

Reopening as this is still present in develop and in #127.

Let's try and address this as part of #119.