Instead of using glade_widget_depends() which implied N^2 invocations/iterations
(where N is the numbers of objects in the project) we now calcualte
dependencies based on property references.
This way we only have to iterace over every object once to check the list
of properties that constitute a reference to them.

In a real world example, sorting objects in geany.glade decreased from 120ms to just 1ms

plugins/gtk+/gtk+.xml.in, plugins/gtk+/glade-gtk-widget.c:
  Removed unused glade_gtk_widget_depends()