Support the strikethrough and its attribute in span tag

Support the strikethrough and its attribute in span tag

The attributes of strikethrough :
  - s-color

How to apply it in TextEditor:

  textEditor.SetProperty(Dali::Toolkit::TextEditor::Property::TEXT, "<span s-color='blue'> hello world span tag </span>nothing<s color='green'> hello world s tag </s> ");
  textEditor.SetProperty(Dali::Toolkit::TextEditor::Property::ENABLE_MARKUP, true);

This patch should be preceded by the patch below:
https://review.tizen.org/gerrit/c/platform/core/uifw/dali-toolkit/+/272018

Change-Id: I87bdf96565a22d90d9f1d1000cb719fbfd6dbc84