Adding additional object properties to serialized JSON

To add additional object properties when serializing objects with JSON.stringify(obj) and loading with canvas.loadFromJSON(json):

1. properties should be returned from toObject method of fabric.Object: https://github.com/kangax/fabric.js/blob/master/src/shapes/object.class.js#L803 or from toObject method of a more specific "class".

2. properties should be added to stateProperties array of fabric.Object: https://github.com/kangax/fabric.js/blob/master/src/shapes/object.class.js#L707 or to stateProperties of a more specific "class".

Example of adding "lockMovementX", "lockMovementY", "lockRotation", etc.

 /**
     * Returns an object representation of an instance
     * @method toObject
     * @return {Object}
     */
   toObject: function(propertiesToInclude) {
      var NUM_FRACTION_DIGITS = fabric.Object.NUM_FRACTION_DIGITS,

          object = {
            type:                     this.type,
            originX:                  this.originX,
            originY:                  this.originY,
            left:                     toFixed(this.left, NUM_FRACTION_DIGITS),
            top:                      toFixed(this.top, NUM_FRACTION_DIGITS),
            width:                    toFixed(this.width, NUM_FRACTION_DIGITS),
            height:                   toFixed(this.height, NUM_FRACTION_DIGITS),
            fill:                     (this.fill && this.fill.toObject) ? this.fill.toObject() : this.fill,
            stroke:                   (this.stroke && this.stroke.toObject) ? this.stroke.toObject() : this.stroke,
            strokeWidth:              toFixed(this.strokeWidth, NUM_FRACTION_DIGITS),
            strokeDashArray:          this.strokeDashArray ? this.strokeDashArray.concat() : this.strokeDashArray,
            strokeLineCap:            this.strokeLineCap,
            strokeLineJoin:           this.strokeLineJoin,
            strokeMiterLimit:         toFixed(this.strokeMiterLimit, NUM_FRACTION_DIGITS),
            scaleX:                   toFixed(this.scaleX, NUM_FRACTION_DIGITS),
            scaleY:                   toFixed(this.scaleY, NUM_FRACTION_DIGITS),
            angle:                    toFixed(this.getAngle(), NUM_FRACTION_DIGITS),
            flipX:                    this.flipX,
            flipY:                    this.flipY,
            opacity:                  toFixed(this.opacity, NUM_FRACTION_DIGITS),
            shadow:                   (this.shadow && this.shadow.toObject) ? this.shadow.toObject() : this.shadow,
            visible:                  this.visible,
            clipTo:                   this.clipTo && String(this.clipTo),
            backgroundColor:          this.backgroundColor,
            fillRule:                 this.fillRule,
            globalCompositeOperation: this.globalCompositeOperation,
            transformMatrix:          this.transformMatrix ? this.transformMatrix.concat() : this.transformMatrix,
            skewX:                    toFixed(this.skewX, NUM_FRACTION_DIGITS),
            skewY:                    toFixed(this.skewY, NUM_FRACTION_DIGITS),
	    selectable:   this.selectable,
		lockMovementX:this.lockMovementX,
		lockMovementY:this.lockMovementY,
		lockScalingX: this.lockScalingX,
		lockScalingY: this.lockScalingY,
		lockUniScaling: this.lockUniScaling,
		lockRotation: this.lockRotation
          };

...

 stateProperties:  (
      'top left width height scaleX scaleY flipX flipY originX originY transformMatrix ' +
      'stroke strokeWidth strokeDashArray strokeLineCap strokeLineJoin strokeMiterLimit ' +
      'angle opacity fill fillRule globalCompositeOperation shadow clipTo visible backgroundColor ' +
      'alignX alignY meetOrSlice skewX skewY selectable lockMovementX lockMovementY lockScalingX lockScalingY lockUniScaling lockRotation'
    ).split(' '),