Fix errors reported by Eclipse on attempting to load Umbrello XMI file into PapyrusUML

umbrello/umlwidgets/associationwidget.cpp
- In function saveToXMI remove duplicate writing of attribute "xmi.id"
  which is already written by WidgetBase::saveToXMI().
- As a cosmetic change, in function loadFromXMI handle case (scene == 0)
  first for saving an indentation level in the nominal execution path.

umbrello/umlwidgets/classifierwidget.cpp
- In function saveToXMI remove duplicate writing of attribute
  "showstereotype" which is already written by UMLWidget::saveToXMI().

umbrello/umlwidgets/umlwidget.cpp
- As a followup to commit e66f6529, in function saveToXMI write
  attribute "showstereotype" unconditionally.

umbrello/version.h
- Increase XMI2_FILE_VERSION to "2.0.1" due to changed save format.