Freeze break: gnome-shell: left over application name in top bar

[Owen Taylor <otaylor redhat com>]

If you switch an empty desktop in GNOME Shell, the application menu
on the top bar mistakenly shows the name of the last application
you had active on the previous workspace.

 https://bugzilla.gnome.org/show_bug.cgi?id=645734

It can also occur after you've closed an application and ended up with
an empty desktop.

This is a fairly recent regression introduced when we were fixing
a blink on the application name prior to 2.91.92. I'd like to commit
the last patch on that bug - think it's very safe and high cost-benefit.

- Owen