add Quadtree.remove (#227)

Co-authored-by: Jonathan Ellis <jbellis@gmail.com>

orx-quadtree/src/commonMain/kotlin/IQuadtree.kt

@@ -35,6 +35,14 @@ interface IQuadtree<T> {
      */
     fun insert(element: T): Boolean
 
+    /**
+     * Remove the given element
+     *
+     * @param element
+     * @return true if the element was present
+     */
+    fun remove(element: T): Boolean
+
     /**
      * Finds which node the element is within (but not necessarily belonging to)
      *