Added undo/redo feature

* new file : HistoryManager.js
* can undo (ctrl-z) and redo (ctrl-y) changes performed through the
tools
* history states are recorded per frame
* a new state is recorder each time a tool is released (introduced
TOOL_RELEASED event for this purpose)
* a duplicated frame doesn't inherit the history states of the original
frame
* there is no limit to the number of states that can be stored per frame
* actions such as creating/duplicating/deleting a frame are not
concerned by this update

js/HistoryManager.js

@@ -0,0 +1,40 @@
+(function () {
+	var ns = $.namespace("pskl");
+	ns.HistoryManager = function () {};
+
+	ns.HistoryManager.prototype.init = function () {
+        document.body.addEventListener('keyup', this.onKeyup.bind(this));
+        $.subscribe(Events.TOOL_RELEASED, this.saveState.bind(this));
+	};
+
+    ns.HistoryManager.prototype.saveState = function () {
+      piskel.getCurrentFrame().saveState();
+    };
+
+	ns.HistoryManager.prototype.onKeyup = function (evt) {
+      if (evt.ctrlKey && evt.keyCode == 90) { // CTRL + Z
+        this.undo();
+      }
+
+      if (evt.ctrlKey && evt.keyCode == 89) { // CTRL+ Y
+        this.redo();
+      }
+    };
+
+    ns.HistoryManager.prototype.undo = function () {
+      piskel.getCurrentFrame().loadPreviousState();
+      this.redraw();
+    };
+
+    ns.HistoryManager.prototype.redo = function () {
+      piskel.getCurrentFrame().loadNextState();
+      this.redraw();
+    };
+
+    ns.HistoryManager.prototype.redraw = function () {
+      piskel.drawingController.renderFrame();
+      piskel.previewsController.createPreviews();
+    };
+
+    ns.HistoryManager = new ns.HistoryManager();
+})();