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.Gui/GuiSystem.cs

@@ -54,7 +54,7 @@ namespace MonoGame.Extended.Gui
         private Screen _activeScreen;
         public Screen ActiveScreen
         {
-            get { return _activeScreen; }
+            get => _activeScreen;
             set
             {
                 if (_activeScreen != value)