Add wxWebView::IsBackendAvailable()

This allows to check if a specified backend has been registered before creation of a new instance

interface/wx/webview.h

@@ -427,6 +427,14 @@ public:
     static void RegisterFactory(const wxString& backend,
                                 wxSharedPtr<wxWebViewFactory> factory);
 
+    /**
+        Allows to check if a specific backend is currently available
+
+        @since 3.1.4
+    */
+    static bool IsBackendAvailable(const wxString& backend);
+
+
     /**
         Get the title of the current web page, or its URL/path if title is not
         available.