Don't crash when background listing outlasts file manager list job

When the QDir listing in the background thread takes a long time,
e.g. by artificially introducing a sleep statement therein, then
it can happen that the FileManagerListJob gets destroyed before
the background task finishes. If that happens, then we used to
access the dangling this pointer to access m_aborted or to invoke
the handleResults method, both of which are undefined behavior.
The latter usually crashes.

Prevent this from happening by introducing a mutex that gets locked
while we are running a background task and during destruction of
the list job. This synchronizes the two tasks and prevents the
crash from happening.