Add wxWebView::SetZoomFactor(float) and GetZoomFactor()

The new method allows to set the zoom level more precisely than the
existing SetZoom(wxWebViewZoom).

Also improve the webview sample by using radio menu items instead of
check items and manually resetting them.

Closes https://github.com/wxWidgets/wxWidgets/pull/1894

Closes #18769.

src/gtk/webview_webkit2.cpp

@@ -913,6 +913,10 @@ wxWebViewZoom wxWebViewWebKit::GetZoom() const
     return wxWEBVIEW_ZOOM_LARGEST;
 }
 
+float wxWebViewWebKit::GetZoomFactor() const
+{
+    return GetWebkitZoom();
+}
 
 void wxWebViewWebKit::SetZoom(wxWebViewZoom zoom)
 {
@@ -944,6 +948,11 @@ void wxWebViewWebKit::SetZoom(wxWebViewZoom zoom)
     }
 }
 
+void wxWebViewWebKit::SetZoomFactor(float zoom)
+{
+    SetWebkitZoom(zoom);
+}
+
 void wxWebViewWebKit::SetZoomType(wxWebViewZoomType type)
 {
     WebKitSettings* settings = webkit_web_view_get_settings(m_web_view);