permit selection of filters available in loaded reports

[dericed]

to reproduce:

1. open qctools. in preferences enable only cropdetect and signalstats.
2. open test.mkv ffmpeg -f lavfi -i testsrc2 -f lavfi -i sine -t 10 test.mkv
3. let analyze and save report
4. close file
5. in preferences enable only signalstats (disable cropdetect)
6. open test.mkv and report will load
7. in "setup filters" pop-up, the cropdetect values are not listed although they are in the report