A TranslationUnit is owned by TranslationUnits now. TranslationUnits
allows to add another TranslationUnit and to update/query the recently
and previously parsed translation unit.

This does not change any behavior yet.

Change-Id: I8a2f0cc05d3e51bf739dd5d7c4da14b54147f3ab
Reviewed-by: David Schulz <david.schulz@qt.io>