Check if edge webview runtime is available in the call to
wxWebView::IsBackendAvailable() instead of only doing it during
process initialization. This allows an application to install
the Edge WebView2 Runtime during runtime and use the edge webview
afterwards without restarting the process.
Setting EnableContextMenu and EnableAccessToDevTools where
ignored before the webview loaded content. This would often
be set directly after creation and work like this with other
backends.
This results in a single intelligible error instead of a hundred of less
clear ones in case our code is compiled against an out of date SDK
version.
There doesn't seem to be any way to check the version directly, so just
check that the interface which hadn't been defined in the previous
version is defined now to check for it indirectly.
