Add a MatroskaJS choice API

This API is meant for interactive movies. The author adds choices in a chapter and retrieves the user choice at the end of this chapter.

Depending on the choice, different actions can be performed.

The choice rendering and user interaction is the responsibility of the player.

chapter_codecs.md

@@ -96,6 +96,58 @@ The command is called with `LogMsg( "A String" );`.
 The `Matroska Player` **SHOULD** send the provided message to the user debugging console, if there is one.
 In this example it would send "A String", without the double quotes, to the debugging console.
 
+### AddChoice
+
+The command is called with `AddChoice( "<ChoiceUID>", group = Null );`.
+
+The `Matroska Player` **MUST** keep the choice with the given UID in memory until the Chapter it belongs to
+has ended. The string may be any length and should be a valid ECMAScript string literal.
+
+If a group string is supplied, the choice **MUST** only be evaluated within the group of the given string.
+Otherwise the choice **MUST** be evaluated with the other choices with no group, called the Default Group.
+
+It is possible to add a single choice for a group to let the user select it or not, for example with a checkbox.
+
+### SetChoiceText
+
+The command is called with `SetChoiceText( "<ChoiceUID>", "Some text", "<lang>" );`.
+
+Set the string to use for the choice with the given UID if the given language is selected.
+The language string is the same form defined in [@!RFC5646] as for the `ChapLanguageBCP47` element.
+
+When the `Matroska Player` collects the strings to use for all available choices, the same language
+rules apply as of [@!Matroska, section 19] for track selection. In addition if a string is missing for a language
+the whole language is considered to be not available, unless there is no other language option available.
+
+<!-- TODO: allow attaching a choice to a tag, we will need a special tag target for that -->
+
+### SetChoiceDefault
+
+The command is called with `SetChoiceDefault( "<ChoiceUID>", group = Null );`.
+
+Tell the `Matroska Player` that the choice with the given UID is the default one to use.
+If this function is not called, no choice is considered the default one.
+
+If a group string is supplied, the default state only applies within the group of the given string.
+Otherwise the default state only applies among the choices of the Default Group.
+
+### CommitChoices
+
+The command is called with `CommitChoices( );`.
+
+Tell the `Matroska Player` that all previously added choices with AddChoice ((#addchoice)) should be used.
+This **SHOULD** generate some visually visible choice, with the default choice selected, if there is one.
+
+### GetChoice
+
+The command is called with `GetChoice( group = Null );`.
+
+The function returns the UID string corresponding to the choice selected by the user.
+If the user did not make a selection and there is no default one, this function returns the ECMAScript `Undefined`.
+
+If a group string is supplied, the selected UID string returned is the selection within the choices of the given group string.
+Otherwise selected UID string returned is the selection within the choices of the Default Group.
+
 
 
 ## DVD Menu