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.
Every time I read this, I wonder whether we shouldn't push this test higher up.  How expensive is `DomUtils.getVisibleClientRect` in the common case?

If we walked the DOM, then we could do this test as we go along.  Do you know whether walking the DOM is slower then:

    document.documentElement.getElementsByTagName "*"