* docs/reference/: Update API docs.

	* libxfce4menu/libxfce4menu.c: Set path pointer to NULL after freeing
	  it to avoid random data in it.
	* libxfce4menu/xfce-menu-merger.c: Prepend a default layout element
	  to the root menu so that the root menu and its children have a
	  layout to use as a fallback. Remove empty layout elements so that
	  we can fallback to the default layout.
	* libxfce4menu/xfce-menu-node.{c,h}: Add new function
	  xfce_menu_node_tree_get_child_node(). Add
	  XFCE_MENU_NODE_TYPE_DEFAULT_LAYOUT to the XfceMenuNodeType enum.
	* libxfce4menu/xfce-menu-parser.c: Parse <DefaultLayout> elements.
	  Attributes are currently ignored.
	* libxfce4menu/xfce-menu.c: Properly look up the right layout or
	  default layout in xfce_menu_get_elements().
	* tests/test-display-menu.c, tests/test-menu-spec.c: Improve error
	  messages.
	* POTFILES.in: Remove deleted file xfce-menu-layout.c.