convenience helper for checking if an entity is inactive

I figure this is fine to add since InactiveComponent is a so-called
"system" component

src/com/blarg/gdx/entities/Entity.java

@@ -1,6 +1,7 @@
 package com.blarg.gdx.entities;
 
 import com.blarg.gdx.entities.systemcomponents.EntityPresetComponent;
+import com.blarg.gdx.entities.systemcomponents.InactiveComponent;
 
 // Yes, this class SHOULD be marked final. No, you ARE wrong for wanting to subclass this.
 // There IS a better way to do what you were thinking of doing that DOESN'T involve
@@ -48,4 +49,8 @@ public final class Entity {
 		else
 			return null;
 	}
+
+	public boolean isInactive() {
+		return entityManager.hasComponent(InactiveComponent.class, this);
+	}
 }