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/chapter_command.cpp

@@ -37,11 +37,6 @@ typedef enum {
 
 namespace mkv {
 
-chapter_codec_cmds_c::~chapter_codec_cmds_c()
-{
-    delete p_private_data;
-}
-
 void chapter_codec_cmds_c::AddCommand( const KaxChapterProcessCommand & command )
 {
     auto codec_time = FindChild<KaxChapterProcessTime>(command);