Add asynchronous debugger using ThreadWeaver, add documentation dock

- Rename old class Debugger to DebuggerAgent and use it in the new
Debugger class, which handles asynchronous commands and provides
signals to inform about results
- Implement asynchronous debugger jobs as derivates of
ThreadWeaver::Job
- Add documentation dock to TimetableMate, shows the HTML
documentation generated by completiongenerator
- Add output directory for the generated HTML documentation with
a CMakeLists.txt and used images
- Fix timetablemate/src/CMakeLists.txt: Use new options of
completiongenerator, add all output files to the list of output