Use flex and bison via FindFLEX and FindBISON on Windows

As discussed, we don't want to build flex and bison ourselves. Rather we just assume they are installed, and try to find and use them from CMake.

Possible ways of installing them:
1) chocolatey https://chocolatey.org/packages/winflexbison
2) vcpkg (although ports don't contain them at the moment)
3) Using Qt Online installer, which would download and install the binaries (currently this does not exist).