Stop link hints from overlapping and detect clickable elements on google inbox

This PR
* rewrites the link hints XPath as DOM code, giving better accuracy and readability
* detects overlapping clickable elements and attempts to find locations where they do not overlap to place a link hint
  - this requires elements to be in the order they appear in the DOM, another reason for using the DOM rather than an XPath
* integrates a variation on PR #1316 to detect clickable elements on google inbox
* allows for multiple images to use the same `<map>` to provide clickable regions
* add support for `<area shape="circle">` in image maps (previously the code assumed everything was a rectangle)
* document the fact that we don't properly support `<area shape="poly">` in image maps
* support clicking `opacity: 0` elements: some websites (notably Facebook) use these to provide the functionality of less attractive elements (eg. `<input type="file">`) while showing an image behind that appears to the user to be the actual button.

This fixes #1307.

Edit: also
* doesn't create link hints for off-screen image maps.
We're only using the rect to work out where our link hint marker should go. Since we only have one link hint per element, we only need one rect.