Reduce Indexing by heuristics

Indexing is quite expensive, so the goal for an usable indexing engine is to avoid it if nothing changes. If we can detect that we don't need to re-index that file. But not only file changes can change the index, project file changes too. To make it more complicated the preprocessor can change the index in a hard to detect way too. We have to find a compromise between usability and speed in that cases. For that we have to document the support and unsupported cases.