GUI option to select devices.txt file#20

Preferably this would be remembered across program launches, like the rest of the settings. I'm not yet sure of the best way to do this. It's definitely an unusual thing to do (most devices are already in the default list), but at the same time, the `--piclist` command line option isn't convenient for Windows users (or Mac users or Linux users for that matter). So I think that it should be present, but it shouldn't be on the main window, but off in a sub-window. Perhaps the About dialog could be made into a fully-fledged "Options & About" dialog. It would mean some additional code but it would be much more versatile. Alternately, the About button could be made into a menu with several options. Yet a third option is to have a Settings tab at the bottom, alongside Target Programming and Data Xfer.