Switch from Travis to GitHub Actions

[BenWiederhake]

Why?

Travis tries to extort Open Source: "Beg for mercy, or receive no more builds" https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

For those of you who have been building on public repositories (on travis-ci.com, with no paid subscription), we will upgrade you to our trial (free) plan with a 10K credit allotment (which allows around 1000 minutes in a Linux environment). Trial plans expire when you use all of our credits or after a year.

Even this "begging for mercy" doesn't seem to work:

At the moment, credit allocation for OSS projects is on hold as per directives from management. Sincere apologies please.

More context:

• https://news.ycombinator.com/item?id=25338983
• https://github.com/SerenityOS/serenity/pull/3980

Proposed solution: Github Actions

This PR implements a "Github Actions workflow", by just moving all we would do on Travis over to GA.

• We need only a single workflow: "Just build and run and test it!"
• This workflow consists of a single job: "Just build and run and test it!"
• A single step in this job is called a step.

Controversial stuff

libtgvoip doesn't work easily

But I also couldn't get it to run on Travis, so this is not a downgrade.

[ars3niy]

What happens if you don't add these things to .gitignore?

[BenWiederhake]

Normally: Nothing.

But if someone runs the ga_build_td.sh script (the thing that's only really useful on GitHub Actions) on their on machine, they might start wondering where those intermediate files came from.

I also prefer to mention "taken filenames that don't appear in git" in the gitignore, so that's another reason.

Is there an issue with it? :)