FileChooser: Sort directories before files by default

This imho is what most people[1][2] would expect to be the default in modern times; considering this also cannot be "easily" configured outside of the cli utilities gsettings or dconf it should just be the default.

If there is any particular reason this has been false by default for such a long time, please let me know!

I can make a MR against gtk-3-24 branch as well later if this is accepted :)

[1] https://superuser.com/questions/843615/gtk-3-14-file-chooser-folders-first
[2] https://askubuntu.com/questions/789505/can-i-choose-to-not-sort-directories-before-files-on-file-open-dialogs