Fix crash when toggling the menu-bar visibility

Use a queued connection to prevent that the menu
gets hidden during it is still open. Thanks to
Christoph Feck for the hint!

Frank and I could not reproduce the issue in our
environments even when following the steps from
bug 278366, however Christoph's hint sounds sane
and let's hope it works (leaves a bad gut feeling
to "fix" bugs without being able to verify them).

BUG: 286822
CCBUG: 278366
FIXED-IN: 4.7.4