The custom UI plugin (v_repExtCustomUI), 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]