Some fixes for Doxygen

- CMake-options `DOCS_GENERATE_DIAGRAMS`, `DOCS_GENERATE_CALL_GRAPHS`, `DOCS_GENERATE_COLLABORATION_GRAPHS` were removed because they are automatically switched to on when the `dot`-tool is found
- simplified `dot`-tool CMake stuff
- Sidebar was not visible
- Namespace docs were not correct
- Some warnings were fixed