* libnautilus-extensions/nautilus-global-preferences.c:
(global_preferences_create_dialog),
(global_preferences_register_sidebar_panels_preferences_for_ui),
(global_preferences_get_sidebar_panel_key),
(nautilus_global_preferences_is_sidebar_panel_enabled),
(global_preferences_register_for_ui),
(user_level_changed_callback),
(nautilus_global_preferences_startup):
Added define for sidebar panels namespace.

Rename preference defines to make it clear whether they are keys
or namespaces.

Add function to obtain a sidebar panel key from an iid.

Use "sidebar panels" consistently in both code and preferences
dialog.

* libnautilus-extensions/nautilus-global-preferences.h:
New public function
nautilus_global_preferences_is_sidebar_panel_enabled () that
returns whether a sidebar panel is enabled.

* src/ntl-uri-map.c: (sidebar_panel_preference_is_on):
Use nautilus_global_preferences_is_sidebar_panel_enabled() instead
of hard-coding the namespace string.