[BugFix] Text variable name is displayed instead of its content.

[Issue]
Text variable name is displayed instead of its content.

[Cause]
  EditText's variables are removed at DisplayListActor#unhookVariable().
  However the variables belong to the parent MovieClip, not to the EditText.
  So, they must keep existing even when the text field is removed from the stage.

[Fix]
  hookVariable() and unhookVariable() are deleted.
  Instead, addVariableListener() and removeVariableListener are introduced.