where possible (everywhere except Windows) replace copying game file
with symlinking, on Linux do the same to CEF libraries.

Remove bundled CEF modules, leave only FindCEF for CEF_ROOT var
handling.

TRy to find system CEF module first.

Linux handlers and main() were copied from the cefsimple test app,
persistent storage implementation rewritten using cstdio and
std::filesystem to make it cross-platform.

Resync build scripts with Chromium 89 and target VS2019 instead of VS2015 so we can use C++17 std::filesystem.