Update code and build to set explicitly the kim-api symbol visibility

[ellio167]

This is a low priority, but can be considered a "best practice". The idea is to set symbol visibility so that only the public parts of the api are visible at link time. This makes it so that users (who have access to the implementation code) cannot use (link to) non-public parts of the api implementation in their code.

Exactly how best to achieve this is not clear. It seems like a set of preprocessor definitions is a good way to go. But exactly what those should be is not obvious.

CMake has some support/awareness of this issue and it would be best to find a "CMake approved" approach to this, so that it doesn't depend on the compilers and/or system on which the build is happening.

Here are some links that may be useful:
https://stackoverflow.com/questions/39981491/c-symbol-visibility-in-static-archives

https://stackoverflow.com/questions/17080869/what-is-the-cmake-equivalent-to-gcc-fvisibility-hidden-when-controlling-the-e

https://phabricator.kde.org/T11490

https://libcork.io/0.15.0/visibility.html

https://dev.to/vejmartin/dll-symbol-visibility-in-c-36ip

https://gist.github.com/ax3l/ba17f4bb1edb5885a6bd01f58de4d542