Add support for WebVTT cue identifier line

Modified the mkvmuxer to write the ChapterStringUID
sub-element of the Chapter Atom element.

Modified the mkvparser to read the ChapterStringUID
sub-element of the chapter atom.

Modified the vttdemux app to write the Cue Identifier
line of the WebVTT cue.

Change-Id: I06fe386f44897ada3fe10cbf89096df104dcf779

webmids.hpp

@@ -118,6 +118,7 @@ enum MkvId {
   kMkvEditionEntry            = 0x45B9,
   kMkvChapterAtom             = 0xB6,
   kMkvChapterUID              = 0x73C4,
+  kMkvChapterStringUID        = 0x5654,
   kMkvChapterTimeStart        = 0x91,
   kMkvChapterTimeEnd          = 0x92,
   kMkvChapterDisplay          = 0x80,