Notify when there are updates

[acrosman]

Summary
While support for auto-update is a ways off (requires lots of setup with OS providers to properly support), we could add a notification when the project itself has a new release.

Please describe the problem you are trying to solve.
Knowing that you're using an out-of-date version to encourage updates.

Proposed Solution
Check github for a new release tag, and pop-up a message saying a new version is ready for download including a link to the release page.