De-select all button for "shown" queries

[danydoerr]

Thanks a lot, Anton, for fixing #141!

I've been using the graph search a lot in the past days and was about to report some of these bugs that you now fixed, but you preempted me!

I have a tiny feature request: When loading FASTA files with lots of records where only few are of interest, it would come in handy to have a "de-select" button for "shown" queries in Step 2 of the graph search screen.