API extension: add tracks with custom IDs, remove tracks, and more

This adds a `generate_ids` parameter to `musly_jukebox_addtracks` to control whether track IDs are to be generated automatically (as before) or provided by the user (closing #2). Furthermore, it adds `musly_jukebox_removetracks`, `musly_jukebox_trackcount` and `musly_jukebox_maxtrackid`.

This also adds a self-test executable for basic unit testing and integration testing of the library. It is compiled if `BUILD_TEST` is set in the cmake compilation options, and can be run stand-alone or via `ctest`.

Finally, it bumps the version number to 0.2, for the next release, and updates the HTML documentation included in the repository.