Refactor ItemRepository mutex handling

This is a collection of patches that remove the deprecated QMutex::Recursive usage from KDevelop and ports some ItemRepository code over to non-recursive mutices.

Basically we now always supply the mutex externally to the ItemRepository instead of having either the m_ownMutex or an externally provided one. The type of the mutex is also not hardcoded to QMutex but can be QRecursiveMutex instead as needed in a few places. Many other areas are ported to non-recursive mutices.

Note for reviewing: The split of ItemRepository into ItemRepositoryPrivate is temporary and will be undone at a later stage of the patch series again.