Fix setting focus to wxWebViewEdge

It was impossible to give focus to the actual web view in wxWebViewEdge
by keyboard navigation or programmatically with wxWebViewEdge::SetFocus().

Fix it by calling CoreWebView2Controller::MoveFocus() in the wxWebViewEdge's
wxEVT_SET_FOCUS handler.

Closes https://github.com/wxWidgets/wxWidgets/pull/2444

include/wx/msw/webview_edge.h

@@ -109,6 +109,8 @@ private:
 
     void OnSize(wxSizeEvent& event);
 
+    void OnSetFocus(wxFocusEvent& event);
+
     void OnTopLevelParentIconized(wxIconizeEvent& event);
 
     bool RunScriptSync(const wxString& javascript, wxString* output = NULL) const;