Add notice for new version

[rpwoodbu]

There's an API for checking if a new version is available. Since users will often leave the app running for long periods at a time, they might be missing out on new features and fixes without realizing it. Look into ways of surfacing this information and letting them restart the app. Possibilities include:

• Show a notice on the "new session" window.
• Use the Notification API to pop up a notice.
  • But be careful to do this in a way that's not annoying.

[rpwoodbu]

Added a notice on the "new session" window in SHA c16be1b. Added a desktop notification separately in SHA 3039301, which makes it easy to rollback if it ends up being obnoxious. See the commit message on the latter for my rationale.