Use Case documents

As agreed in meeting today I have created a first draft of a consolidated
UC document at https://www.w3.org/2017/dxwg/wiki/Use_Cases_and_Requirements

This document will hold UC's as agreed by the group, and proposals under
development for voting by the UC sub-group.

Where a UC is voted subject to edits, the edited version has status "open"
- where it is yet to be voted on it will be "New"

At this point I have removed individual contributor details - as these will
often represent multiple raw suggestions - but this can be changed by the
will of the group if necessary :-)

These are first cuts and i may have transcribed poorly - and i'm operating
on a single laptop screen here so too easy to miss things - please feel
free to edit formatting. Could content changes be run past the UC sub-group
however.

Rob Atkinson