Followup to commit 9d9800c3 fix for crash on exit with tabbed diagrams

There was a further crash on closing in the code path
  UMLApp::slotFileClose() => slotFileNew() => m_doc->newDocument()
due to DiagramsModel::m_views not having been reset to empty.
This caused access to stale UMLView objects already deconstructed.

umbrello/models/diagramsmodel.{h,cpp}
- New function removeAllDiagrams clears content of m_views.

umbrello/uml.cpp
- In function slotFileClose call diagramsModel()->removeAllDiagrams()
  on m_doc before calling slotFileNew().

CCBUG: 430322