A mix between Tiled and GUI stuff (#553)

* mostly restored the tiled importer unit tests

* code cleanup

* code cleanup

* almost got tiled maps serialization independent of the content pipeline

* refactored the tiled map content importer so that tiled maps can be loaded at runtime

* code cleanup

* removed nuspec files

* experimenting with an XML based GUI markup parser

* a few more markup parser features

* finally got a working open file dialog

* experimenting with markup bindings

* attached properties

* working on the gui stuff again

* working on a better textbox

* multiline textboxing

* new text box is really coming along

* removed the content explorer experiment

* restored the old gui demo back to the way it was before (for now)

Source/MonoGame.Extended.Tiled/Serialization/TiledMapObjectLayerContent.cs

@@ -0,0 +1,28 @@
+using System.Collections.Generic;
+using System.Xml.Serialization;
+
+namespace MonoGame.Extended.Tiled.Serialization
+{
+    public class TiledMapObjectLayerContent : TiledMapLayerContent
+    {
+        public TiledMapObjectLayerContent()
+            : base(TiledMapLayerType.ObjectLayer)
+        {
+            Objects = new List<TiledMapObjectContent>();
+        }
+
+        [XmlAttribute(AttributeName = "color")]
+        public string Color { get; set; }
+
+        [XmlElement(ElementName = "object")]
+        public List<TiledMapObjectContent> Objects { get; set; }
+
+        [XmlAttribute(AttributeName = "draworder")]
+        public TiledMapObjectDrawOrderContent DrawOrder { get; set; }
+        
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}