Spannable: Add BoldSpan

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