Plugin repo API version check & warning

[ilonachan]

This is a future-proofing measure: if, according to my proposal, an api version field is attached to plugins to manage breaking changes to the plugin API, older versions of the app should be able to detect and reject plugins for newer API versions. No sophisticated logic is required for now: as long as the very first stable 2.0 release has this feature, users won't be confused once plugin version starts actually being used later.