As a consequence, we can remove code in CMake files to handle these
dependencies on `gtest` and `gmock`, which fixes the following error
when running CMake:

```sh
$ cmake -G Ninja ../.. -DPython_EXECUTABLE=python3.7 -DBUILD_TESTS=ON -DENABLE_COVERAGE=ON -DBUILD_CPP_DOCS=ON -DBUILD_PYTHON_DOCS=ON
...
CMake Error at CMakeModules/testing_cpp.cmake:47 (IF):
  if given arguments:

    "WIN32" "AND" "STREQUAL" "Debug"

  Unknown arguments specified
Call Stack (most recent call first):
  common/cpp/src/common/CMakeLists.txt:11 (add_plain_unit_test)
```

Added project strucre; some tests & first producer

changed folder structure

Made test build optional

Changed project structure for producer

added doxygen

added doxygen to cmake

fixed doxygen file

made googletest separated

changed cmake strucure

Added head lock guard