[NUI] Add SetTextFit, GetTextFit to TextLabel

[NUI] Add SetTextFit, GetTextFit to TextLabel

Add a TextFit struct to pass data of DALi TextFit PropertyMap.
The TextFit struct can be used as an argument to
SetTextFit and GetTextFit methods.

// example
var textFit = new Tizen.NUI.Text.TextFit();
textFit.Enable = true;
textFit.MinSize = 10.0f;
textFit.MaxSize = 100.0f;
textFit.StepSize = 5.0f;
textFit.FontSizeType = FontSizeType.PointSize;
label.SetTextFit(textFit);

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