Enable GitHub Actions #200

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