One-line fix for Tiled failures on GWT. (#7176)

* One-line fix for Tiled failures on GWT.

It seems like Tiled now uses reflection for more types than just MapObject in the `com.badlogic.gdx.maps` package, so this preemptively adds all of that package to the reflection cache, like how `com.badlogic.gdx.maps.objects` and `com.badlogic.gdx.maps.tiled.objects` already are added wholesale.

* Subpackages don't need to be specified.

This also corrects the DTD link, since I don't think the http one works anymore, and it was for the wrong version.

* It looks like more was needed!

backends/gdx-backends-gwt/res/com/badlogic/gwtref/GwtReflect.gwt.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE module PUBLIC "-//Google Inc.//DTD Google Web Toolkit trunk//EN" "http://www.gwtproject.org/doctype/2.8.0/gwt-module.dtd">
+<!DOCTYPE module PUBLIC "-//Google Inc.//DTD Google Web Toolkit trunk//EN" "https://www.gwtproject.org/doctype/2.8.2/gwt-module.dtd">
 <module>
 	<inherits name='com.google.gwt.user.User' />
 	<define-configuration-property name="gdx.reflect.include"
@@ -48,13 +48,35 @@
 	<extend-configuration-property name="gdx.reflect.include"
 		value="com.badlogic.gdx.Net" />
 
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.Map" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.MapGroupLayer" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.MapLayer" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.MapLayers" />
 	<extend-configuration-property name="gdx.reflect.include"
 		value="com.badlogic.gdx.maps.MapObject" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.MapObjects" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.MapProperties" />
 	<extend-configuration-property name="gdx.reflect.include"
 		value="com.badlogic.gdx.maps.objects" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.tiled.TiledMap" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.tiled.TiledMapImageLayer" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.tiled.TiledMapTileLayer" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.tiled.TiledMapTileSet" />
+	<extend-configuration-property name="gdx.reflect.include"
+		value="com.badlogic.gdx.maps.tiled.TiledMapTileSets" />
 	<extend-configuration-property name="gdx.reflect.include"
 		value="com.badlogic.gdx.maps.tiled.objects" />
-	
+
 	<extend-configuration-property name="gdx.reflect.include"
 		value="com.badlogic.gdx.graphics.g3d.particles.ParticleEffect" />
 	<extend-configuration-property name="gdx.reflect.include"