Parse layout properties

Parse layout properties

If a widget has a LayoutManager instance, then we want to parse layout
properties in UI description files; the grammar is similar to packing
properties in GtkContainer:

    <child>
      <object ...>
        <property name="...">...</property>
        <layout>
          <property name="pname">value</property>
        </layout>
      </object>
    </child>

The properties are applied after a child has been added to its parent,
to the parent's layout manager property should be set.