Add the ability to add version requirements to extensions

[atcuno]

Either before parity or, if not, right after, we need the ability to version either:

1. extension classes

OR

2. exposed methods within extension classes

Right now, these are holes as a method in an extension class can change - which plugins depend on - but the plugin has no way to mark this. This will lead to API breakage, which is what we want to avoid and the whole reason for versioning.

@ikelos I am assigning this to you to think what system you want for this. We can then overhaul all the extensions at once (I assume they would all start 1,0,0 since none have it now.

References from the GUI ticket:

• Add APIs and initial plugins for GUI support and fit Windows major APIs to current design flow #1640 (comment)