[FileFilter] Create HotSpot's for files in child dirs too

Match files in subdirs of the current dir too, this enables opening
files from the output of e.g. grep.

Also use QSet::cbegin/cend to prevent detaching (if the container is not
const, calling begin/end on it will make it detach).

This partially reverts 0b489b0c69b8ae2f.