[css-flexbox] Accessibility of Order

I have added a general example of the reason why the tab order needs to be
updated to match the visual order when it is changed via Flexbox Order. In
turn, the accessibility API should also get the same visual flow.

https://wiki.csswg.org/spec/css3-flexbox/accessibility

I would like to continue understanding any further arguments stopping this
proposal since PF is asking for this, IBM and Freedom Scientific agree this
is what needs to happen, and I believe I have addressed the arguments made
so far in the wiki document.

/bo