Add check for updates feature

It's hard to know if UTM is up to date if it's been downloaded from outside the Mac App Store. (one could watch the GitHub releases, but that requires a GitHub account)

There should be an automatic and manual way of checking for updates.

In the Mac App Store version, the feature could either be excluded or we could have it link to `macappstore://showUpdatesPage?scan=true`, which opens the Mac App Store and checks for updates.