Wholesome CI (Win/Mac)

[simonsan]

With the amount of upcoming changes we probably should add a .travis.yml as a temporary solution (as long as our own solutions aren't ready) to check if nyan is building on MacOSX. In addition I would also propose to use appveyor (and integrate it next to Kevin and Travis) to check PRs for win32/64.

For sure this is not the cleanest solution, but it is better than not having any CI for both platforms and fight against broken builds.

• [X] Grant Organisation Access to Appveyor
• [x] Activate nyan repository in Travis options
• [x] ~~Register organization account with Appveyor to integrate with Github checks~~
• [x] ~~grant GitHub-Team access~~
• [x] ~~Move CI from personal account to this organisation account~~
• [ ] Move from Travis to Github Workflow for Win and MacOSX

Update: With Github Workflows and Actions we could get away from Travis and Appveyor also here (SFTtech/openage#1196).

[simonsan]

We can use MSYS2 (#80) on a Windows Github Workflow, which might be much easier to setup.