Implement Item#setParent, making Item#parent writeable. Closes #186.

src/item/Item.js

@@ -977,11 +977,43 @@ var Item = this.Item = Base.extend(Callback, {
 	 *
 	 * // Now the parent of the path has become the group:
 	 * console.log(path.parent == group); // true
+	 * 
+	 * @example // Setting the parent of the item to another item
+	 * var path = new Path();
+	 *
+	 * // New items are placed in the active layer:
+	 * console.log(path.parent == project.activeLayer); // true
+	 *
+	 * var group = new Group();
+	 * group.parent = path;
+	 *
+	 * // Now the parent of the path has become the group:
+	 * console.log(path.parent == group); // true
+	 * 
+	 * // The path is now contained in the children list of group:
+	 * console.log(group.children[0] == path); // true
+	 * 
+	 * @example // Setting the parent of an item in the constructor
+	 * var group = new Group();
+	 * 
+	 * var path = new Path({
+	 * 	parent: group
+	 * });
+	 * 
+	 * // The parent of the path is the group:
+	 * console.log(path.parent == group); // true
+	 * 
+	 * // The path is contained in the children list of group:
+	 * console.log(group.children[0] == path); // true
 	 */
 	getParent: function() {
 		return this._parent;
 	},
 
+	setParent: function(item) {
+		return item.addChild(this);
+	},
+
 	/**
 	 * The children items contained within this item. Items that define a
 	 * {@link #name} can also be accessed by name.

test/tests/Item.js

@@ -63,6 +63,41 @@ test('addChild(item)', function() {
 	},  1);
 });
 
+test('setting item.parent', function() {
+	var layer1 = paper.project.activeLayer;
+	var layer2 = new Layer();
+	layer1.activate();
+	var group = new Group();
+
+	var path = new Path();
+	equals(function() {
+		return path.parent === layer1;
+	}, true, 'Path is a child of layer1 because it is active');
+
+	path.parent = layer2;
+	equals(function() {
+		return path.parent === layer2;
+	}, true, 'The parent of path was set to layer2');
+
+	path.parent = group;
+	equals(function() {
+		return path.parent === group;
+	}, true, 'The parent of path was set to group');
+	equals(function() {
+		return layer2.children.indexOf(path) === -1;
+	}, false, 'The path is no longer a child of layer2');
+
+	var path2 = new Path({
+		parent: group
+	});
+	equals(function() {
+		return path2.parent === group;
+	}, true, 'setting the parent in the constructor');
+	equals(function() {
+		return group.children.indexOf(path2) == 1;
+	}, true, 'the index of path2 is 1, because group already contains path from before');
+});
+
 test('item.parent / item.isChild / item.isParent / item.layer', function() {
 	var project = paper.project;
 	var secondDoc = new Project();