GUI: Allow user to display hidden files in the browser dialog.

This implements feature request #3600774 "File browser: show hidden files".

Short preview on how it looks: http://lordhoto.users.sourceforge.net/scumm-browser-hidden.png
I am not sure whether I'm too happy with the checkbox placement, but I have no better idea where to put it...

Link to the feature request: https://sourceforge.net/tracker/?func=detail&atid=418823&aid=3600774&group_id=37116