Buildscripts/cmake uninstall target

[LostInKadath]

CMake doesn't support any make uninstall commands by default. This may be embarrassing for users, switching from building with autotools. This PR adds a special uninstall target to do the deinstallation process.

During cmake --install step, an install_manifest.txt file is created, which contains a list of installed files -- binaries, libraries, headers and so on. The uninstall target calls cmake -E rm for every file listed in this manifest. Simple and handy.

To launch the deinstallation process, one can run:

cmake --build .builddir --target uninstall

The information in README.md is updated. An additional workflow is added, which smoke-tests the installation/deinstallation process.