Some improvements for the NSOpenPanel/NSSavePanel

I made some changes in the Open and Save panels used for the configuration.

- By default, hidden files are hidden: it's quite unpleasant to see all the hidden files in the panel but it's a personal opinion. The user can show/hide the hidden files by using command shift period (default OS X feature in NSOpenPanel / NSSavePanel).

- The last accessed file url is persistent. A bookmark is used, so even if the file moves, the next time, the url will stay valid and will reflect the new path.