github: Migrate from Travis CI to Github Actions

On travis-ci.com (travis-ci.org will be discontinued by the end of the
year) we are now charged for each minute.  We only got 10000 credits in
a trial plan, which we used up with a few builds.  Minutes also cost a
different amount of credits on different platforms: 10 on Linux,
but 50 on macOS (installing the dependencies on macOS alone took 12-15
minutes on Travis for some reason, takes about half on Github's runners).

No native Windows build yet as we have the same issue as on AppVeyor where
threading/streaming tests might get stuck.  And there is also only a
single Windows platform to test on.  Plus building/testing on Windows is
very slow (and getting ccache to work seems tricky).

The 'sw_collector' test case had to be disabled because we can't access
/usr/local/share on the Github build hosts (the process is just blocked
in readdir() and eventually times out).

Unfortunately, we can't test on different architectures anymore (in
particular ARM and the big-endian IBM Z/x390x).

.github/actions/default/action.yml

@@ -0,0 +1,16 @@
+name: "Default CI Build Steps"
+runs:
+  using: "composite"
+  steps:
+    - name: "Install Dependencies"
+      run: ./scripts/test.sh deps
+      shell: bash
+    - name: "Install Python Dependencies"
+      run: ./scripts/test.sh deps
+      shell: bash
+    - name: "Build Dependencies"
+      run: ./scripts/test.sh build-deps
+      shell: bash
+    - name: "Build/Tests"
+      run: ./scripts/test.sh
+      shell: bash