demux: mkv: separate Chapter Codec implementations

And move most UI/Button data in the dedicated thread. So it doesn't block the UI.

modules/demux/mkv/demux.cpp

@@ -25,8 +25,7 @@
 #include "demux.hpp"
 #include "stream_io_callback.hpp"
 #include "Ebml_parser.hpp"
-
-#include <vlc_actions.h>
+#include "virtual_segment.hpp"
 
 namespace mkv {