Use iterator instead of collecting all the files to search beforehand.
Especially for the file system filter, it's better not to block while collecting all the file names by first iterating over the file system. Now, the filters return an iterator, and the search thread takes a file from it, searches it, and takes the next file. This also unifies the file iterator for the custom locator filters and the find in files on file system search filter. Task-number: QTCREATORBUG-1690