Intercepting element clicks

Hi,

The chromedriver, at least, will notify users if they attempt to click on
an element that isn't actually clickable (eg. because it's covered by
another) There's an open PR (#299
<https://github.com/w3c/webdriver/pull/299>) that adds a new "element
intercepting click" for this, but there's some discussion on that diff
concerning how this should be implemented.

Rather than leaving the PR languishing in purgatory, we should have a
discussion here. The choices that I see are:

* Ignore this case, and leave it unspecified.

* Optionally allow drivers to raise an "invalid element state" error if the
user would not actually be able to click on the element (which may make
testing for clickjacking impossible since we don't have any language to say
whether or not an element would be user visible any more)

* Add a new error message, and work out the spec-ese to make this happen.

I'm assuming that this only applies to "Element Click", since the advanced
user interactions are working on coordinates rather than elements, and so
should be figuring out the element which should receive events.

If we had the time, the last option feels like the best, with users being
forced to use the Action commands to test clickjacking. Thoughts?

Simon