Add UhkBuffer.backtrack() and implement the KeyAction class.

model/KeyAction.ts

@@ -0,0 +1,30 @@
+/// <reference path="KeystrokeAction.ts" />
+
+class KeyAction {
+
+    static fromJsObject(jsObject: any): KeyAction {
+        switch (jsObject.actionType) {
+            case 'keyStroke':
+                let keystrokeAction = new KeystrokeAction();
+                keystrokeAction.fromJsObject(jsObject);
+                return keystrokeAction;
+            default:
+                throw 'Unknown KeyAction actionType "${jsObject.actionType}"';
+        }
+    }
+
+    static fromBinary(buffer: UhkBuffer) {
+        let keyActionFirstByte = buffer.readUInt8();
+        buffer.backtrack();
+
+        if (KeystrokeAction.firstValidScancode <= keyActionFirstByte &&
+            keyActionFirstByte <= KeystrokeAction.lastValidScancode)
+        {
+            let keystrokeAction = new KeystrokeAction();
+            keystrokeAction.fromBinary(buffer);
+            return keystrokeAction;
+        } else {
+            throw 'Unknown KeyAction first byte "${keyActionFirstByte}"';
+        }
+    }
+}