It didn't make sure its Function* stayed valid. Now it does, by keeping
a copy of the list of CppDocuments (the snapshot) around.

Reviewed-by: Roberto Raggi

Some people don't like this.

It is now possible to choose between case-sensitive or case-insensitive
completion. Also, you can now disable the automatic insertion of braces.

This change removes a number of wrong usages of the CppModelManager::documents()/document().

The code completion was keeping around references to old documents after
the completion finished. This caused documents to stay in memory when
unloading projects, up until the next time you used the completion.