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.
> That means any win of 11% or more by filtering non-visible elements earlier is an overall win.

Unless you know a way to detect which elements are floated or absolutely positioned before time, we still need to process every element, except for those situations I mentioned in [an earlier comment](https://github.com/philc/vimium/pull/1355#issuecomment-67732765). Since these are not common, I don't think we can regularly expect to hit your 11% target.