do not export Context_xxx of master elements

It can be retrieved with EBML_CLASS_CONTEXT(), no need to specifically export them
from DLLs.
Neither mkvtoolnix nor VLC use these, apart from Context_KaxMatroska,
which doesn't have a definition outside of here.

matroska/KaxContexts.h

@@ -8,25 +8,12 @@
 #ifndef LIBMATROSKA_CONTEXTS_H
 #define LIBMATROSKA_CONTEXTS_H
 
-#include "matroska/KaxTypes.h"
+#include "matroska/KaxConfig.h"
 #include <ebml/EbmlElement.h>
 
 namespace libmatroska {
 
 extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxMatroska;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxSegment;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxAttachments;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxAttached;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxChapters;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxCluster;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxTags;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxTag;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxBlockGroup;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxCues;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxInfo;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxSeekHead;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxTracks;
-extern const libebml::EbmlSemanticContext MATROSKA_DLL_API Context_KaxTrackEntry;
 
 extern MATROSKA_DLL_API const libebml::EbmlSemanticContext & GetKaxGlobal_Context();