* libxfce4menu/xfce-menu-directory.{c,h}: Add function

	  xfce_menu_directory_equal() which compares two XfceMenuDirectory
	  objects based on their GFile members.
	* libxfce4menu/xfce-menu-layout.{c,h}: Use GList instead of GSList
	  everywhere.
	* libxfce4menu/xfce-menu-merger.{c,h}: Never access GNode data
	  directly. Instead always use the xfce_menu_node_tree*() functions.
	* libxfce4menu/xfce-menu-node.{c,h}: Add functions
	  xfce_menu_node_tree_get_child_nodes(),
	  xfce_menu_node_tree_get_string_children(),
	  xfce_menu_node_tree_get_boolean_child() and
	  xfce_menu_node_tree_get_string_child() to collect the values of
	  immediate children of a GNode tree.  Add new functions
	  xfce_menu_node_tree_set_string() and
	  xfce_menu_node_tree_set_merge_file_filename() so that GNode data
	  needs to be accessed less often. Fix assertions in a few other
	  functions.
	* libxfce4menu/xfce-menu-parser.c: Fix segmentation fault due to an
	  uninitialized GError in GIO.
	* libxfce4menu/xfce-menu.{c,h}: Use GList instead of GSList everywhere.
	  Improve API docs for a few public functions. Change xfce_menu_new()
	  again to expect a filename as the first parameter. Add a separate
	  function xfce_menu_new_for_file() to create an XfceMenu from a
	  GFile. Rename xfce_menu_get_root() to xfce_menu_new_applications().
	  Replace a lot of traversing code with the new
	  xfce_menu_node_tree_get*() functions.
	* tests/test-display-menu.c, tests/test-menu-parser.c,
	  tests/test-menu-spec.c: Update to new libxfce4menu API.