builder-tool: Set the parent of newly created GtkAssistantPage

builder-tool: Set the parent of newly created GtkAssistantPage

...and its property. Otherwise simplify_element() blows up when looking
up the hierarchy to determine a property type.

  $ gtk4-builder-tool simplify --3to4 /dev/stdin <<EOF
  > <?xml version="1.0" encoding="UTF-8"?>
  > <!-- Generated with glade 3.20.2 -->
  > <interface domain="nm-applet">
  >   <object class="GtkAssistant">
  >     <child>
  >       <object class="GtkBox" id="confirm_page">
  >       </object>
  >       <packing>
  >         <property name="page_type">confirm</property>
  >         <property name="complete">True</property>
  >       </packing>
  >     </child>
  >   </object>
  > </interface>
  > EOF
  /dev/stdin: Packing property GtkAssistant::page_type not found
  /dev/stdin: Packing property GtkAssistant::complete not found
  Segmentation fault (core dumped)