Add a Javascript Image type for offscreen image preloading in QML

It would be very nice to provide an Image type which can be used in Javascript like:

var ctx = canvas.getContext('2d');
var img = new Image();
img.src='images/image.gif';
img.onload = function() {
   ctx.drawImage(img);
}

This feature is very helpful for porting existing 3rd party Canvas 2D/3D applications to QML.