CMake split into lib and project files

[ulrikus]

What

Based on the changes in the branch msvetkin/feature/cmake-split-project-and-lib, but with a bit more extracted to the top level CMakeLists file. See https://github.com/msrd0/QtWebApp/pull/33 to the base repo. Also, a simple variable declaration for better readability in the httprequest.cpp file.

[msrd0]

Hi, thanks for your contribution. Unfortunately I haven't been using this project neither C++ for a very long time, so I'm not interested in maintaining this project anymore.