The custom UI plugin (simExtCustomUI), courtesy of Federico Ferri, offers functionalities based on the Qt framework. The source code to that plugin can be found here.
The custom UI plugin offers Qt-style widgets such as dialogs integrating buttons, edit-boxes, sliders, labels, images, etc. Any action on custom UIs (e.g. button-click, text edition, slider movement) is reported as a script callback. Other functions are accessed via related API function calls. This allows customizing a simulation to a great extent. Following displays a typical custom UI:
[Custom UI example]