Finalize the engine rebranding

Spring is clearly dead, all sane people migrated from there, so by now the naming is more a liability than advantage (if were were advantages to start with). This is low prio, but good to have gradually:

### TODO
- [ ] Duplicate all `Spring.` calls with `Recoil.` calls in Lua space. All new call outs should use `Recoil.` semantic.
- [ ] Change all titles from Spring to Recoil in various places, that are displayed to the user.
- [ ] Duplicate output binary names, release both `recoil*` and `spring*` binaries.
- [x] Leave Spring engine versioning (will probably stay forever as 105 and/or BAR105), and add another versioning scheme for Recoil.
- [ ] Change namespaces and function names within the engine to use Recoil
- [ ] The comments saying "this file is part of Spring project", can this be changed or only amended? In any case do so.
- [ ] Not strictly rebranding related, but would be great to update start menus to use new UI (esp after RmlUI work is merged). The current one is quite horrible.
- [ ] This one is under the question mark. https://github.com/beyond-all-reason has been a great place for us and there were no conflicts of interest as Recoil treated all of its users fairly I think, still maybe a good move is to at least mirror the repo to https://github.com/recoilengine, so people know it's not only a BAR engine.

No files or tests are named. Start by breaking the TODO list into separate areas—Lua calls, user-facing titles, binaries, engine namespaces and comments—and inventory where each is implemented; done means the agreed Recoil naming is applied consistently, while the start-menu and repository-mirroring questions are resolved separately.

Written by the indexing model from the issue text.