Bug 13252: Do not store data in the app bundle

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

The first time an updated browser is opened, migrate the existing
browser profile, Tor data directory contents, and UpdateInfo to the
TorBrowser-Data directory. If migration of the browser profile
fails, an error alert is displayed and the browser is started
using a new profile.

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

Add support for installing "override" preferences within the user's
browser profile. All .js files in distribution/preferences (on
Mac OS, Contents/Resources/distribution/preferences) will be copied
to the preferences directory within the user's browser profile when
the profile is created and each time Tor Browser is updated. This
mechanism will be used to install the extension-overrides.js file
into the profile.

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.