Bug #4234: Use the Firefox Update Process for Tor Browser.

New configure options:
  --with-tor-browser-version=VERSION # Pass TB version throughout build.
  --enable-tor-browser-update        # Enable bundle update behavior.
The following files are never updated:
  TorBrowser/Data/Browser/profiles.ini
  TorBrowser/Data/Browser/profile.default/bookmarks.html
  TorBrowser/Data/Tor/torrc
Mac OS: Store update metadata under TorBrowser/UpdateInfo.
Removed the %OS_VERSION% component from the update URL (13047) and
  added support for minSupportedOSVersion, an attribute of the
  <update> element that may be used to trigger Firefox's
  "unsupported platform" behavior.
Windows: disable "runas" code path in updater (15201).
Windows: avoid writing to the registry (16236).
Also includes fixes for tickets 13047, 13301, 13356, 13594, 15406,
  16014, and 16909.