When using this method with a string argument, the method would need
to be queried every time via QMetaObject::indexOfMethod. Using a
functor one can get rid of this, saving a few cycles which can
add up when handling many inotify events.

Note that the benchmark timing does not really improve significantly.
Using a profiler like perf, we do see that less cycles are consumed
though. In my measurement, this reduces the cpu cyles by about 2%.