Remove dependency on boost's binary libraries ( header-only libraries are still in use )

* Using Qt's QDir instead of boost::path,
+ introduced 'getInstallSubdir' helper function that will return QDir initialized to specific subdirectory of the installDir.
* Also, using QT5's CMake qt5_use_modules instead of explicit linking - @ReDucTor can You check if this breaks anything on Your side ?
* added '-Wall' and '-Wextra' to g++/clang++ options