Add wxWebView::GetBackendVersionInfo()

Allows to get the WebView backend version if available
Currently implemented for IE, Edge and webkit2.

include/wx/msw/private/webview_edge.h

@@ -67,6 +67,7 @@ public:
 
     static wxDynamicLibrary ms_loaderDll;
     static wxString ms_browserExecutableDir;
+    static wxString ms_version;
 
     static bool Initialize();