General `sandboxed-tor-browser` UI/UX improvement catch-all.

The sandboxed-tor-browser UI/UX is something that was done by someone that is not a UI/UX person, and it shows. There is tons of room for improvement, and this is the catch-all for tracking such things.

Off the top of my head:

• Allow the ability to localize it. (https://github.com/nicksnyder/go-i18n looks promising).
• Use a progress bar instead of a spinner for the progressDialog.
• Add an About dialog somewhere so that it's easy to query version information, without resorting to the command line (--version).
• Improve the appearance of the error dialog, provide access to the log when convenient (rather than the console, or a log file).