[EEZ Studio] Allow user widget creation from selected elements in the widget structure

[jcuypers]

Allow for the creation of a user widget based of the selection of widgets in the widget structure

Work flow:

1. User selects widgets in the widgets structure
2. Right click and selects build user-widget
3. EEZ studio prompts for a user widget name
4. A user widget is created with this name
5. All widgets are duplicated with the exact same properties and styles (and actions)
6. the size of the user widget is automatically calculated and set based on the extents of the selected widgets