Should handle differing Pinta versions

[robpvn]

https://bugs.launchpad.net/pinta/+bug/1575875
https://bugs.launchpad.net/pinta/+bug/1580205

These show that we have a versioning problem. Some add-ins might legitimately require a newer version (like 1.7), while others will work fine for Pinta 1.6 and newer. If we have a subrepo for each version Pinta can just look in those, but then add-ins have to be re-added for every new version. We need to look at possibilities for keeping the repos like they are but letting Pinta check for compatability.

[robpvn]

If we are adding subrepos for every version, we can improve the tooling to simplify the process by creating a little C# program that uses the Mono.Addins libs powering mautil directly. For example, the user could run something like mono AddPintaAddin.exe mynewaddin.mpack --platform=All --versions=1.6,1.7 and it would do the rest.

I guess the proposed AddPintaAddin would deserve its own GitHub repo.

[cameronwhite]

I thought we did have versioned repos, but maybe I'm thinking of the old add-ins server or confusing it with the platform-specific repos

[robpvn]

The old add-in server did have versioned repos, yeah. But it was a beast to maintain otherwise and was costing me 10$ a month - the GitHub version has been much easier to keep running! My bad that I didn't foresee this situation when I first set up this version, I should probably have had versioning from the start.