Add boeken/.. files as content/resource
Catalog is a singleton, now. Seems ... easier.
Broke a few unit-tests, fixing
Undo/redo commands -> UndoAbleCommand
I DIDN'T DO IT
Make instance publicly settable

So it can be disposed and afterwards reset, as well
Propagate SelectedProduct to SelectedProducts
Transaction isolationlevel change, dispose catalog
Add linq reference to tests
remove unused empty lambda's
cleanup, add assertions, add test