Allow overriding protected methods on sfMusic and sfSoundStream

[Marioalexsan]

Fixes #394.

Implements overriding for protected methods in sfMusic and sfSoundStream by using the pattern described here.

The Chunk struct was also updated since the data types were different from the ones used in the C++ library:

// CSFML old struct
typedef struct
{
    int16_t*     samples;     ///< Pointer to the audio samples
    unsigned int sampleCount; ///< Number of samples pointed by Samples
} sfSoundStreamChunk;

// SFML's struct
struct Chunk
{
    const std::int16_t* samples{};     //!< Pointer to the audio samples
    std::size_t         sampleCount{}; //!< Number of samples pointed by Samples
};

Additionally, ci.yml was updated so that a newer version of CMake would be fetched. mirroring the current code in the SFML repo. Without this, CI would fail due to an unsupported old version (< 3.5) being fetched.