The HTMLLinkElement caller is a bit hairy, so we shove an await() in
there temporarily. This is sure to cause fun times for anyone debugging
task/microtask execution order.
This replaces the previous Web::ImageDecoding::Decoder interface.
While we're doing this, also move the SerenityOS implementation of this
interface from LibWebView to WebContent. That means we no longer have to
link with LibImageDecoderClient in applications that use a web view.
