Bug 922558: add PredicateContext to sdk/context-menu

[petli]

As noted over on https://bugzilla.mozilla.org/show_bug.cgi?id=922558, everything but documentType is implemented.

[Mossop]

I presume mimetype, node.ownerDocument.contentType should be it, but check what that is when viewing images

[petli]

That made sense. For images, it's e.g. image/jpeg.

[petli]

Comments addressed with two additional patches. I verified that this merges cleanly onto current master.

[petli]

Please review last commit that moves error handling into selection.js. This turns out to be mostly correct behaviour, the DOM code just throws a slightly incorrect exception. See e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=746906

(I don't know why I didn't try to catch the exception the first time...)

[Mossop]

Awesome thanks. Good work finding the spec for that behaviour too, I had missed it. Since there's a few intermediate commits here I've gone ahead and squashed the branch and landed this as 8f80dfc