Fix crash loading XMI

Do assign parent when loading XMI. Commit 1bfad17b regressed the loading of (for
example) RefactoringAssistant.xmi. This fix makes loadFromXmi() only determine
a new parent when pasting (so, when not loading a file) like it was before.

CCBUG: 70926
CCBUG: 279788
CCBUG: 195803