Integrate selected suggestions from...

Integrate selected suggestions from https://mail.kde.org/pipermail/umbrello-devel/2021-December/023006.html

umbrello/umlwidgets/widgetbase.{h,cpp}
- Add const qualifier at functions hasDocumentation, useFillColor,
  autoResize, changesShape.
- Add const overloads to functions asActivityWidget to asUMLWidget.

umbrello/umlwidgets/widgetbase.cpp
- In operator=(const WidgetBase&) guard against self-assignment.
- In function boundingRect change type of halfWidth to qreal.

THANKS
- Add Robert Hairgrove.