Support the underline and its attributes in span tag

 The attributes of underline :
  - u-color
  - u-height
  - u-type
  - u-dash-gap
  - u-dash-width

 How to apply it in TextLabel:

  textLabel.SetProperty(Dali::Toolkit::TextLabel::Property::TEXT, "Before <span u-color='blue' u-type='dashed' u-height='4.0f' u-dash-gap='2.0f' u-dash-width='3.0f'> Hello World</span> after.");
  textLabel.SetProperty(Dali::Toolkit::TextLabel::Property::ENABLE_MARKUP, true);

Change-Id: Ia3dfd19e0f7a40903785e6d93d2587b9bfcac056