Use QSet to search for strings instead of QStringList

The time that it takes to search a collection of just too many strings
can take up to 48msec - tested triggering the code on /usr/lib with
around 7000 files.

changing to QSet this went down to 35msec. This is not a lot but
it's triggered at *every* mouse move event, so the gain is cummulative

The whole experience is a bit smoother.