Avoid crash when InsetBibitem::updateCommand shows a dialog

Avoid crash when InsetBibitem::updateCommand shows a dialog

This is one of the places where a dialog is shown (indicating that
some bibtems have been renamed) while the metrics are not up to date.
Then a draw operation can be triggered too early and a crash can ensue.

Use ad-hoc solution for this case and use Buffer::setBusy(). The
Alert::warning helper cannot really do that since it doe snot know the
current buffer.