Generate assests on merge using Github actions

[ccordoba12]

@dpizetta mentioned in https://github.com/ColinDuquesnoy/QDarkStyleSheet/pull/253#issuecomment-761851591:

As the process of generating those resources is not automatized yet, I would recommend we use the develop branch to PR's, so we can keep the fully functional code in the master (for Python and C++).

I think I could help with that by using Github actions to generate our assests when a pull request is merged against deveolp and commit them to the repo.

@dpizetta, what do you think?

[dpizetta]

@ccordoba12 of course, essentially we need to run the script process_qrc. Tests and other ones are ok, see here.

We are using Tox to run the processes so we can use the same configuration locally on our machines to test. tox.ini has all possible configurations and .travis.yml has those to run over the server.

The build command on tox (pyqt5-build: python example/example.py --qt_from=pyqt5 --test ) also generates the screenshots we see in docs/README.

Some steps we need to generate we need:

• [ ] Associate CI/CD to PR (require pass);
• [ ] Run and fix build (py38-pyqt5-build) over the server - we had some problems with deps;
• [ ] Save resources generated in CI/CD into the repository (update resources);

If you or another one can help me it would be great. Tks