Merge branch 'bug-9961-fix-qch-file-handling'
- bug-9961-fix-qch-file-handling: Make the context sensitive help (F1) work for CTK plug-ins. Refined handling of (un)registration of .qch files. Unregister legacy BlueBerry plugin .qch files before registering them. Use absolute path for unregistering .qch files. Use a directory relative to the original .qhc file for the cache.