Use only one "directory contents counting" thread per process

Use only one "directory contents counting" thread per process

The pointer to QThread object is stored in a global variable, and each
view increments/decrements a reference count when it starts/stops using
the thread. If this thread reaches zero, the thread is stopped.

Note that we cannot just use a smart pointer, like QSharedPointer, to
manage the thread, because we must make sure that the thread is not
running any more before the QThread is deleted.

REVIEW: 115064