Input fields and refactors required to support them

TestGame/TestGame.cs

@@ -73,7 +73,7 @@ public class TestGame : Game
 
         _uiSystem.SetStyleSheet(_styleSheet);
 
-        var addButton = new Button("Default button", _defaultFontSet);
+        var addButton = new Button("", _defaultFontSet);
 
         var removeButton = new Button("Danger button", _defaultFontSet);
 
@@ -93,10 +93,16 @@ public class TestGame : Game
             Anchor = Anchor.TopCenter
         };
 
+        var inputField = new InputField(string.Empty, _defaultFontSet)
+        {
+            PlaceholderText = "Hello, World!"
+        };
+
         c.AddChild(addButton);
         c.AddChild(removeButton);
         c.AddChild(outlineButton);
         c.AddChild(linkButton);
+        c.AddChild(inputField);
 
         var vc = new VerticalContainer(0.0f);
         vc.AddChild(c);