Bug 13252: Do not store data in the app bundle

When --enable-tor-browser-data-outside-app-dir is enabled,
all user data is stored in a directory named
TorBrowser-Data which is located next to the application directory.

Display an informative error message if the TorBrowser-Data
directory cannot be created due to an "access denied" or a
"read only volume" error.

On Mac OS, add support for the --invisible command line option which
is used by the meek-http-helper to avoid showing an icon for the
helper browser on the dock.