This should simplify the merging of the improved lookup context.

Introduced pp-scanner.cpp, renamed `pp' to `Preprocessor' and removed useless #includes.

The method `preprocess' returns the preprocessed text for the given expression.

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

recover from invalid asserts, so it will crash anyway, but not in
a controlled environment.

the evaluator of bit-or expressions.

Task:     - 234321
RevBy:    - Bjoern

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.

The scope of the classes is now separated and displayed in the second
column, and not included in the search.