CefExecuteProcess needs the app pointer otherwise initialization in multi-process mode doesn't work correctly

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.

Consolidate the general, platform-agnostic part of the main()/WinMain()
in static FCHost::main() function to avoid code duplication.

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.

Hack a Javascript color picker into FC when it's loaded from FCHost, since we don't have a native one.

Current versions of CEF no longer support CefFrame::LoadString, so switch to a data-URI scheme instead

Probably ought to actually build the storage module, since it's pretty much the most important part of this whole thing.