Windows:  Check validity of this pointer in WebView2 callbacks and free WebView2Loader.dll in destructor

Linux:  Fixed some issues with headless builds and don't terminate process when no X server is running

Windows:  Added WindowsWebView2WebBrowserComponent wrapper to add support for custom WebView2Loader.dll location and user data folder for WebView2

Windows:  Added support for Chromium-based WebView2 browser in WebBrowserComponent and removed WinRT webview