WebViewEdge: Forward window visibility to SDK

Tell the SDK controller if the control is shown or hidden.
This allows creating a hidden webview which will be shown later
and might fix other potential issues.

include/wx/msw/webview_edge.h

@@ -134,6 +134,8 @@ private:
 
     void OnSize(wxSizeEvent& event);
 
+    void OnShow(wxShowEvent& event);
+
     bool RunScriptSync(const wxString& javascript, wxString* output = NULL);
 
     wxDECLARE_DYNAMIC_CLASS(wxWebViewEdge);