Serialize access to the progress bar

The only way https://bugs.kde.org/show_bug.cgi?id=331616&list_id=939077
can happen is if two actions try to initialize, and therefore delete,
the progress bar at the same time. So, serialize with a mutex and at
the same time wrap the progress bar in a QPointer so we can be sure
to be able to figure out whether it has been deleted.

BUG:331616