Add Github Actions CI builds

[ferdnyc]

After discovering that GAMMARAY_CLIENT_ONLY_BUILDs are broken in the repo (#766, fix proposed in #765), I realized that there were no CI builds being done (which would've caught that).

(There are buildbot CI jobs being run against the repo, but they're not public. So for anyone outside of KDAB, there's no way to know what they test. Presumably, not client-only builds! And also not Qt6 builds, based on the job names.)

So, this PR adds Github Actions configuration to run a set of GammaRay build and install tests on every push to, and pull request against, the master branch.

• Both ubuntu-latest and macos-latest platforms are tested
• Two versions of Qt are tested: 5.15.* and 6.4.*
• For each os+Qt, two builds are done:
  1. Using default options
  2. Configured with -DGAMMARAY_CLIENT_ONLY_BUILD=1

These CI builds are EXPECTED TO FAIL, currently. This PR depends on my PR #765, which will fix client-only builds in the master branch.

Other notes

• -DGAMMARAY_PROBE_ONLY_BUILD=1 builds are currently not tested, because they require a more complicated setup involving an existing build tree. However, I do plan to work on an update to the CI configuration that would test them, by using a second build job (not second step) which depends on the first, and uses the existing build trees from the standard build to perform a second probe-only build for a different Qt version. A future PR will add testing of probe-only build runs.

• The CI workflow file incorporates the linuxdeployqt steps from the old Travis CI configuration. However, they are currently disabled with an if: false rule. This is necessary due to the AppImage tools' shortsighted requirement that AppImages be generated on the oldest possible platforms available.

  That platform is currently ubuntu-18.04, which is both (a) deprecated by GitHub, and more importantly, (b) shipped with a glibc version that lacks the GLIBC_2.28 symbols required to build GammaRay's master branch. This makes building an AppImage of GammaRay impossible, based on the rules imposed by the AppImage project.

[CLAassistant]

All committers have signed the CLA.

[dantti]

Thanks, closing in favor of #838