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/TiledMapTileLayerDataChunkContent.cs

@@ -0,0 +1,26 @@
+using System.Collections.Generic;
+using System.Xml.Serialization;
+
+namespace MonoGame.Extended.Tiled.Serialization
+{
+	public class TiledMapTileLayerDataChunkContent
+	{
+		[XmlAttribute(AttributeName = "x")]
+		public int X { get; set; }
+
+		[XmlAttribute(AttributeName = "y")]
+		public int Y { get; set; }
+
+		[XmlAttribute(AttributeName = "width")]
+		public int Width { get; set; }
+
+		[XmlAttribute(AttributeName = "height")]
+		public int Height { get; set; }
+
+		[XmlElement(ElementName = "tile")]
+		public List<TiledMapTileContent> Tiles { get; set; }
+
+		[XmlText]
+		public string Value { get; set; }
+	}
+}