Split update option into 3 prefs

Its been suggested that we should split the "Disable updates during Tor usage" into 3 prefs:

1. Always update over Tor.
2. Never update over Tor.
3. Update over either Tor or Non-Tor.

These options would operate independently of the Torbutton update mechanism.

[Automatically added by flyspray2trac: Operating System: All]