add Construct 3 perpetual license fallback

Can the Scirra team take a look at how JetBrains manage licences. They offer a perpetual license for the previous version for anyone that has the software for at least 12 months. It means that people like myself don't mind paying for a product thats being developed as I get to keep something at the end of it. I'm not saying I'm not willing to pay monthly/yearly but I won't be willing until the product is feature complete which looks like a couple of years yet.

We've talked about this at length. In summary:

• We need to pay for various bits of infastructure that support C3. Users on old versions not upgrading would be an ongoing cost us
• We'd need to patch old versions when breaking changes happen - this can be really difficult
• We'd struggle to offer support to these users. Saying "support only for newest version" doesn't cut it imo.

There'd be a lot of changes and complications our end - all so people wouldn't need to pay any more. It's not something we're looking at doing.