detach() safely in QVector::erase(), and update callers to not detach.

detach() safely in QVector::erase(), and update callers to not detach.

remove() can use non-detaching iterators internally before calling
erase(), which hasn't been exploited so far, so that the detach() in
erase() never actually detached. When using erase() from outside,
you can't do it legally without calling begin() or end() that detach()
before erase() is called.
Now remove() doesn't detach anymore, and detaching in erase() works.
With new tests that fail after changing only the erase() callers
and pass again after fixing erase().

Change-Id: I47c0a9e362dce8628ec566f5437d951755de96c8
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>