Fixed bug 1853 (Want preference for always sorting folders first)

	For now, this is a global preference. Making it
	per-window later is a possibility. Arlo is working on
	a design for directory view defaults that may incorporate
	this.

	* libnautilus-extensions/nautilus-file.h:
	* libnautilus-extensions/nautilus-file.c:
	(nautilus_file_compare_for_sort_reversed): Eliminated this in
	favor of a parameter to nautilus_file_compare_for_sort.
	(nautilus_file_compare_for_sort): Add a parameter for reversed,
	and another for directories_first.
	(nautilus_file_compare_for_sort_internal): Former guts of
	nautilus_file_compare_for_sort moved here.
	(nautilus_self_check_file): Update compare_for_sort self-checks.

	* libnautilus-extensions/nautilus-global-preferences.h:
	* libnautilus-extensions/nautilus-global-preferences.c:
	(global_preferences_install_descriptions),
	(global_preferences_install_defaults),
	(global_preferences_install_visibility),
	(global_preferences_create_dialog): Install an "Always list
	folders before files" preference in the Icon and List Views
	section.

	* src/file-manager/fm-directory-view.h:
	(sort_directories_first_changed): New function pointer for
	subclasses to override.
	* src/file-manager/fm-directory-view.c:
	(fm_directory_view_should_sort_directories_first): New function
	for subclasses to call.
	(sort_directories_first_changed_callback): Call virtual function
	when preference changes.
	(fm_directory_view_initialize): Store initial value of preference;
	install preference callback.
	(fm_directory_view_destroy): Remove preference callback.

	* src/file-manager/fm-desktop-icon-view.c:
	(desktop_icons_compare_callback): Respect preference
	setting.

	* src/file-manager/fm-icon-view.c:
	(icon_container_compare_icons_callback): Update to respect
	directories_first setting.
	(icon_container_compare_icons_by_name_callback): Update for API
	change only.
	(fm_icon_view_sort_directories_first_changed): Override virtual
	function; re-sort icon container if in auto-layout mode.
	(fm_icon_view_initialize_class): Install virtual function override.

	* src/file-manager/fm-list-view.c:
	(fm_list_view_initialize_class): Install virtual function override.
	(list_view_compare_files_for_sort): New function, broken out of
	fm_list_view_compare_rows, and changed to respect directories_first
	setting.
	(fm_list_view_compare_rows): Break out list_view_compare_files_for_sort.
	(compare_rows_by_name): Update for API change only.
	(real_sort_directories_first_changed): Override virtual function;
	re-sort list.
	(fm_list_view_display_pending_files): Update code that sorts
	pending items before adding them to list.
	(fm_list_view_sort_items): Add comment.