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.