umbrello/petaltree2uml.cpp improvements on converting diagram elements:

- Move viewTagToWidget from function umbrellify to global level and
  reset it to empty on beginning parse of diagram.
- In function handleAssocView :
  - Do not fail if attribute "roleview_list" is not found.  Reason:
    InheritView, RealizeView, and UsesView do not have this attribute.
  - If "roleview_list" is not present then retrieve `supplier` and
    `client` using attr->findAttribute() for "supplier" and "client",
    respectively.
  - Move locals supW and cliW up to wider scope in order to attempt
    their retrieval from viewTagToWidget[supplier] and
    viewTagToWidget[client], respectively. Execute the code involving
    view->umlScene()->widgetOnDiagram() only if client and/or supplier
    is not a key in viewTagToWidget.
- In function umbrellify case (dt != Uml::DiagramType::Undefined) :
  - Clear viewTagToWidget.
  - In for-loop over atts, on handling CategoryView/ClassView etc
    retrieve attribute "icon" from attr. If it is "Interface" then
    - set local pointer cw to dynamic_cast<ClassifierWidget*>(w);
    - if cw is non null then set visual property `DrawAsCircle` on cw.

CCBUG: 81364