Fix and improve GitHub Actions CI

* Fixed problem with older compilers by explicitly installing the
  right compiler version.
* Split apart simple builds that can be well described by a build
  matrix, and builds that need special CMake defines or similar
  special snowflake handling.
* Ported some extras + examples builds from TravisCI to GitHub
  Actions.

.travis.yml

@@ -71,15 +71,6 @@ matrix:
       env: COMPILER='clang++-6.0' CPP17=1
 
 
-    - os: linux
-      compiler: clang
-      addons:
-          apt:
-              sources: *all_sources
-              packages: ['clang-8', 'libstdc++-8-dev']
-      env: COMPILER='clang++-8' CPP17=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
-
-
     # GCC builds
     - os: linux
       compiler: gcc
@@ -97,14 +88,6 @@ matrix:
           packages: ['g++-6']
       env: COMPILER='g++-6' CPP14=1
 
-    - os: linux
-      compiler: gcc
-      addons:
-        apt:
-          sources: *all_sources
-          packages: ['g++-7', 'lcov']
-      env: COMPILER='g++-7' CPP14=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
-
 
     # Special builds, e.g. conan
     - language: python