Migrate to GitHub actions from Travis

[TobiGr]

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

This PR supersedes #496 and therefore closes #496.
Thanks to @FireMasterK for doing the initial (and most) work!

I fixed the JDoc deployment and updated the link for Java SE components.
This PR targets master for technical reasons (such as getting the JDoc deployed asap).

To Do / Needs to be fixed later

• Cache builds although the tests fail.

[FireMasterK]

You can't really fix the caching on a build failure, see: actions/cache#92.

However, a workaround is to fork the cache action and then do something like gerbal/always-cache, but that's going to require some maintenance work.

However, it shouldn't be too big of an issue considering these builds last 2 minutes.

[TobiGr]

Yes, you are right.

I'll merge this now. We can improve this later if necessary