support prefetching qml, js, image and plugin resources in QML

Prefetching is a widely used mechanism for better performance and user experience, qml files, javascript files, images file and plugins should be able loaded asynchronously at the application start. For example:

ResourceLoader {
  scripts:["a.js", "b.js", "c.js"]
  images:["a.png", "b.png", "c.png"]
  components:["A.qml", "B.qml", "QtQuick.LocalStorage 2.0"]
}