Create branch work/363582-stable-xmi from master @ df0d4542

- Arguments of functions saveToXMI1 and save1 are changed from
    QDomDocument& doc, QDomElement& elem
  to
    QXmlStreamWriter& writer
- Basic load/save are working but more testing is needed.
  In particular, drag, cut/copy/paste, and undo/redo need testing.
- Unit tests are adapted and are passed.

CCBUG: 363582