Enable GitHub Actions

[s1hofmann]

Short overview
Since Travis effectively dropped support for open source projects we should switch to GitHub Actions

Use case
A working CI setup without having monthly costs.

Detailed feature description
Travis dropped support for open source projects and would require a monthly subscription (70€ / month).
GitHub Actions have several advantages:

• Linux, Windows and macOS builds in a single system, no more mixture of Travis CI and Appveyor
• Faster builds
• No montly cost (at the moment)
• Large amount of community steps

Without a doubt, moving to GitHub Actions looks like a smart move for us.

Additional content

Please provide any (mandatory) additional data for your desired feature

[svettwer]

As a little remark: One could potentially contact travis support and ask them whether they would be so kind to offer monthly open source credits for the project.

https://docs.travis-ci.com/user/billing-faq/#what-if-i-am-building-open-source

But to be honest, I 💯% agree. GitHub Actions is more comfortable and comes with less barriers for open source projects than travis does at the moment.