Fix #8452 (app's crashing for non-existent system catalog)

* make the system catalog a BCatalog instead of a BCatalogAddOn*,
  such that using a non-existing system catalog won't crash but
  simply return the untranslated string instead
* rename MutableLocaleRoster::GetSystemCatalog() to LoadSystemCatalog()
  and adjust it to use BCatalog::SetTo() in order to replace the
  data used by the given catalog
* adjust all users of gSystemCatalog accordingly

headers/private/locale/MutableLocaleRoster.h

@@ -46,7 +46,7 @@ public:
 									// contain the language-name(s)
 			status_t			SetFilesystemTranslationPreferred(bool preferred);
 
-			status_t			GetSystemCatalog(BCatalogAddOn** catalog) const;
+			status_t			LoadSystemCatalog(BCatalog* catalog) const;
 
 			BCatalogAddOn*		LoadCatalog(const entry_ref& catalogOwner,
 									const char* language = NULL,
@@ -109,7 +109,7 @@ struct RosterData {
 
 			BLocale				fDefaultLocale;
 			BTimeZone			fDefaultTimeZone;
-			
+
 			bool				fIsFilesystemTranslationPreferred;
 
 			bool				fAreResourcesLoaded;

headers/private/locale/SystemCatalog.h

@@ -10,7 +10,7 @@
 
 namespace BPrivate {
 
-extern BCatalogAddOn* gSystemCatalog;
+extern BCatalog gSystemCatalog;
 
 }	// namespace BPrivate