Enhancement: Add plugin update checker

[noahc3]

It would be nice to notify the user if there are updates available for APL or plugins. We should build a system for plugins to define an update check and either display a notification to the user when plugin updates are available or just show it in the APL prefs page.

[micahlt]

I've done similar things to this in a few side projects - in my opinion the simplest and least controversial way to do this is by keeping a JSON file somewhere in the repository that keeps track of the versions, then fetching that like an API to compare it to the local versions.