gtkfilechooserwidget: set 'New folder' button focus-on-click

As general rule, all buttons that launch a menu should not grab focus on click, because otherwise when the menu is closed the focus goes back to the button instead of the previously focused widget, which is the one the user was interacting with.

So this is also the case for the 'New Folder' button on the filechooser.

Fixes #2557 (closed)