[libgdx] Add uber jar task for skeletonviewer.

spine-libgdx/build.gradle

@@ -26,6 +26,20 @@ project("spine-libgdx") {
     }
 }
 
+project("spine-skeletonviewer") {
+    jar {
+        duplicatesStrategy = DuplicatesStrategy.INCLUDE
+
+        manifest {
+            attributes "Main-Class": "com.esotericsoftware.spine.SkeletonViewer"
+        }
+
+        from {
+            configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) }
+        }
+    }
+}
+
 configure(subprojects - project("spine-libgdx")) {
     sourceSets.main.resources.srcDirs = ["assets"]