Spannable: Add BackgroundSpan

BackgroundSpan: Span to change the back groundSpan of characters.

Example:
===========================================================================================

    Dali::Toolkit::Text::SpannableString ss =
    Dali::Toolkit::Text::SpannableString::New("Testing");
    auto greenSpan = Dali::Toolkit::Text::BackgroundColorSpan::New(Color::GREEN);
      ss.AttachSpan(
      greenSpan,
      Dali::Toolkit::Text::Range::New(0u, 3u));
    Dali::Toolkit::Text::SetSpannedText(textLabel,ss);

==========================================================================================

Change-Id: I3f5414ee5268799ad8e174db4b693c70f4b9b1b3