Followup to commit 3203b16a work on UML Instances,

> Inheritance cleanup:
> - UMLInstance is derived from UMLObject (this might be changed to
>   UMLCanvasObject in future).

umbrello/uml1model/instance.{h,cpp}
- Derive UMLInstance from UMLCanvasObject.
- Remove function findChildObjectById, use inherited function.
- Remove type AttributeValues and function getAttrValues, use function
  subordinates() of superclass.
- Remove member m_attrValues, it is obsoleted by UMLCanvasObject member
  m_List.

umbrello/umlwidgets/classifierwidget.cpp
- Synchronize function drawMembers with the above changes.

umbrello/umllistview.cpp
- In function connectNewObjectsSlots fix indentation of switch cases.
- Synchronize functions addNewItem and loadChildrenFromXMI with the
  changes in class UMLInstance.

CCBUG: 123633