Image on all platforms

[victorteokw]

On every platform, image should be supported.
However, there are differences between platforms. For web platform, images are fetched from web, for local platform, some are, some are not.

Local images are always part of UI, while web images are always related to content and are dynamic.

On local platform, images gets its width and height just on create, however there is no way to get image's intrinsic size before the image is fetched.

Also, on local platform, different size versions of an image are provided, made it easy to work for different displays.

By using ImageMetadata, on web platform, we can get the behavior of local image.
Get width and height just on create.
Support @2x, default, @3x even much more!

For web images, native platform should support asynchronous loading and resizing.

[victorteokw]

Consider to load image at loading time on web platform.

Maybe optimize and compile the pngs into a big png.