Add option for skipping Tor Launcher during Tor Mail startup

The default behaviour of the Tor Mail bundle is to first start Tor Launcher (which starts Tor) and then start Thunderbird. This is similar to the order of operation in the Tor Browser Bundle.

However, unlike TBB, users may want to start Thunderbird without launching Tor Launcher (and Tor), so that they can use it in offline mode to read/compose emails. Currently, this can be achieved by setting the environment variable TOR_SKIP_LAUNCH to 1, after which Tor Launcher will skip starting Tor and will just start Thunderbird.

We should find a better way of doing this so that users can start Thunderbird without Tor and without setting variables. Perhaps we can have a dialog that prompts the users? Or an option in Tor Launcher?