Modernize project structure and build system

[SpotlightKid]

• docs: convert readme, changelog, authors, and install docs to Markdown
• feat: generate dev requirements via pip-tools
• feat: switch to meson / mesonpep517 build system
• feat: remove travis and Appveyor CI setup
• feat: add GitHub Actions workflow to build wheels with cibuildwheel

[mikacousin]

As you can see in https://github.com/flathub/com.github.mikacousin.olc/pull/10#issuecomment-1483685138, there is something broken with setuptools and flatpak. This PR seems to solve the problem. Do you think it is possible to merge it? Thank you for all your work.