Fix TextAreaHandler in Mac

1. fix `Text`,`TextColor` and `Font`, not need use `AttributedString`, because their value be always take effect in global
2. fix `SelectedText`, because the new SelectedText's color and font was set to default
What will this return if the control is disabled? 
```
var textArea = new TextArea { TextColor = Colors.Blue };
textArea.Enabled = false;
var color = textArea.TextColor;
// color should be Colors.Blue still
```