Revise "Photo - covered" Extractor

* Add support for other covered images(e.g. Twitter, Amazon, Flickr, pixiv, etc.)
* "Photo - covered" Extractor's position should be the next of "Photo - background image" Extractor
* Remove code about "Photo - area element" Extractor
* Add getCoveredImageInfo()
* utility.js: Add getDOMWindowUtils(), getNodesFromPosition()