Cleanup BCatalogAddOn.

* rename BCatalogAddOn to BCatalogData, since it doesn't represent an
  add-on, but rather the catalog data provided by an add-on
* move BCatalogData out of Catalog.{h,cpp} into its own header and
  implementation file
* drop BCatalogData::MarkForTranslation() methods, they're not needed
* drop BCatalog::GetNoAutoCollectString() methods, they're not being
  used anywhere
* cleanup the B_TRANSLATE_... macros somewhat
* add versions of the B_TRANSLATE_MARK_... macros that are meant to be
  used in void context (when the string isn't being used by the program,
  just meant to be picked up by collectcatkeys).
* adjust several apps to use B_TRANSLATE_MARK_..._VOID where needed
* adjust users of BCatalogAddOn accordingly

src/apps/aboutsystem/AboutSystem.cpp

@@ -389,7 +389,7 @@ private:
 AboutApp::AboutApp()
 	: BApplication("application/x-vnd.Haiku-About")
 {
-	B_TRANSLATE_MARK_SYSTEM_NAME("AboutSystem");
+	B_TRANSLATE_MARK_SYSTEM_NAME_VOID("AboutSystem");
 
 	AboutWindow *window = new(std::nothrow) AboutWindow();
 	if (window)