[NUI] Add SetPlaceholder, GetPlaceholder to TextField, TextEditor

[NUI] Add SetPlaceholder, GetPlaceholder to TextField, TextEditor

Add a Placeholder struct to pass data of DALi Placeholder PropertyMap.
The Placeholder struct can be used as an argument to
SetPlaceholder and GetPlaceholder methods.

// example
var placeholder = new Tizen.NUI.Text.Placeholder();
placeholder.Text = "placeholder text";
placeholder.TextFocused = "placeholder textFocused";
placeholder.Color = new Color("#45B39D");
placeholder.FontFamily = "BreezeSans";
placeholder.FontStyle = new Tizen.NUI.Text.FontStyle()
{
    Width = FontWidthType.Expanded,
    Weight = FontWeightType.ExtraLight,
    Slant = FontSlantType.Italic,
};
placeholder.PointSize = 25.0f;
//placeholder.PixelSize = 50.0f;
placeholder.Ellipsis = true;

field.SetPlaceholder(placeholder);

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