Fix file dialog filters
Review Request #127057 - Created Feb. 12, 2016 and submitted
The first one seemed to be still the KFileDialog syntax .. but I only found it by mistake. What I actually wanted to fix was the second one. Having GnuCash (*) in there seems wrong, so I took all known GnuCash extensions there. Only *.gnucash is recommended these days but supporting the older extensions is good for people with older backups.