lib: media_player: split libvlc_media_player_select_track

More than one users reported that they would prefer having a new function to
unselect. Furthermore, having a type and a track of the same type as arguments
feels confusing.
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macos
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nightly-snap

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nightly-win32

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passed #447634
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nightly-win64

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passed #447635
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nightly-win64-llvm

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passed #447639
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