Spannable: Add ItalicSpan

Spannable: Add ItalicSpan

ItalicSpan: Span to change the font slant of characters.

Example:
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    Dali::Toolkit::Text::SpannableString ss =
    Dali::Toolkit::Text::SpannableString::New("Testing italic span");
    auto italicSpan =Dali::Toolkit::Text::ItalicSpan::New();
    ss.AttachSpan(italicSpan,Dali::Toolkit::Text::Range::New(0u, 8u));
    Dali::Toolkit::Text::SetSpannedText(textLabel,ss);

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Change-Id: I9832203ed07433be292ca2dc66d82899ced288ce