Spannable: Add BoldSpan
BoldSpan: Span to change the font weight of characters.
Example:
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Dali::Toolkit::Text::SpannableString ss =Dali::Toolkit::Text::SpannableString::New("Testing");
auto boldspan =Dali::Toolkit::Text::BoldSpan::New();
ss.AttachSpan(boldspan,Dali::Toolkit::Text::Range::New(0u, 3u));
Dali::Toolkit::Text::SetSpannedText(textLabel,ss);
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Change-Id: I583648a7bb850ec36b3dd4ade3d84995113d192f