Optimizations to the import-structure stuff:
- Allow doing "temporary" imports, which is imports that are not recursively propagated into importers(needed in C++).
- Do not maintain the shortest path for now, it's too slow in very complicated cases. Any path is ok.
- Do not compute the recursive imports until they are requested for the first time. This saves us from expensive computations where it's not needed.
- Add mechanisms to import and unimport multiple contexts at a time, for speedup.

All this stuff has a test in kdevelop, and it still succeeds.