Fix macOS WebView script-handler cleanup after wxWidgets 3.3 upgrade

Commit ed88cbe removed `new WebViewWebKit` on macOS because wx 3.3
dropped the no-arg wxWebViewWebKit constructor, falling through to
wxWebView::New(). That bypassed the WebViewWebKit destructor that calls
RemoveScriptMessageHandler("wx"), reintroducing the WebKit teardown bug
the subclass was added to fix.

Restore the macOS-specific subclass path by adding a constructor that
forwards to the wx 3.3 wxWebViewConfiguration-based ctor.

src/slic3r/GUI/Widgets/WebView.cpp

@@ -173,6 +173,12 @@ private:
 
 class WebViewWebKit : public wxWebViewWebKit
 {
+public:
+    WebViewWebKit()
+        : wxWebViewWebKit(wxWebView::NewConfiguration(wxWebViewBackendWebKit))
+    {
+    }
+
     ~WebViewWebKit() override
     {
         RemoveScriptMessageHandler("wx");
@@ -257,6 +263,8 @@ wxWebView* WebView::CreateWebView(wxWindow * parent, wxString const & url)
 
 #ifdef __WIN32__
     wxWebView* webView = new WebViewEdge;
+#elif defined(__WXOSX__)
+    wxWebView* webView = new WebViewWebKit;
 #else
     auto webView = wxWebView::New();
 #endif