[NUI] Add SetFontStyle, GetFontStyle to Text Components

[NUI] Add SetFontStyle, GetFontStyle to Text Components

Add a FontStyle struct to pass data of DALi FontStyle PropertyMap.
The FontStyle struct can be used as an argument to
SetFontStyle, GetFontStyle, SetInputFontStyle and GetInputFontStyle methods.

If a placeholder struct is added later, it can be used as a property of the placeholder.

// example
var fontStyle = new Tizen.NUI.Text.FontStyle();
fontStyle.Width = FontWidthType.Expanded;
fontStyle.Weight = FontWeightType.Bold;
fontStyle.Slant = FontSlantType.Italic;
textLabel.SetFontStyle(fontStyle);

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>