* libnautilus-extensions/nautilus-global-preferences.c:
	(global_preferences_install_one_default),
	(global_preferences_install_defaults),
	(global_preferences_populate_pane), (default_font_callback),
	(default_smooth_font_callback), (default_home_location_callback):
	Simplify the installation of default values.  Use the same
	mechanism as we do for constant default values, but provide a
	callback for dynamic values.

	* libnautilus-extensions/nautilus-preferences.h:
	* libnautilus-extensions/nautilus-preferences.c:
	(preferences_key_make_for_default),
	(preferences_key_make_for_default_getter),
	(nautilus_preferences_set_user_level),
	(nautilus_preferences_get_user_level_name_for_display),
	(nautilus_preferences_get_user_level_name_for_storage),
	(nautilus_preferences_user_level_clamp): New function to clamp a
	user level to the valid values.
	(nautilus_preferences_user_level_is_valid): New function to test a
	user level for validity.

	* libnautilus-extensions/nautilus-sidebar-functions.h:
	Add public define for sidebar panel preferences name prefix.

	* libnautilus-extensions/nautilus-sidebar-functions.c:
	(sidebar_panel_make_preference_key): Make this function private as
	its not used anymore.