Add update checker

[danielchalmers]

Check for updates in the background every time the app is launched and show a persistent blue button below the clip list if any are found that takes you to the latest release page. Auto updates are not necessary.

For testing purposes, we'll always show it since we debug as version 0.0.0, and this allows us to test the feature.

[danielchalmers]

Alternatively, a simple link that takes you to the releases page would be a lot easier to test and maintain. But less cool :(