Add strategy to flatten subclasses as class categories when exporting

[eMaringolo]

As with #39 when loading, we need a strategy to "flatten" an Application Subapps into a single Tonel Package, where each class defined in a Subapp is assigned to a single package and moved into a category.

E.g. MyApplication contains a Core SubApplication, then the classes defined in Core will exported as part of the MyApplication package, with the category MyApplication-Core.

Both the application and subapplication should know how to map to a specific tag if defined (so CoreSubApp could be converted to whatever CoreSubApp class>>_tonelTagName returns (in case there is an App/SubApp prefix defined in #34).