[motion-1] !important: motion-* rename

Hi list,

We resolved during the F2F in San Francisco to rename motion-* to offset-*.

During the F2F in Lisbon, the issue of a collision with the logical
properties ED was raised (offset-inline-start, offset-inline-end,
offset-block-start, offset-block-end). We decided to rename offset-inline-*
and offset-block-* as nobody was shipping them yet.

Yesterday, we learned that Firefox has actually shipped these properties (
https://github.com/w3c/fxtf-drafts/issues/51) and even discussed this with
the WG during a telecon (
https://lists.w3.org/Archives/Public/www-style/2015Jul/0040.html).

I'm left wondering what to do. Our usage counters show that motion-* is
enjoying an increasing level of usage. We have a rename ready to go (and
we've reached out to some of the bigger users to prepare them for it), but
now I don't see how we can legitimately rename without an assurance from
Firefox that they're happy to deal with the fallout when the WG renames the
offset-inline-* and offset-block-* properties.

On the other hand, if we don't rename now, we're exposing ourselves to a
significant risk that we will need to maintain legacy motion-* names. At
this point I'm reluctant to expose us to that risk given that it could have
been avoided entirely if the issue of already-shipped conflicting names had
been brought up at either F2F. Note that this is a *shared risk*, not just
a risk to Chrome, as if we are forced to maintain the aliases there's a
likelihood that you will all need to implement them too.

I think there are three viable options. I'd really like to get a sense for
the appetite of the WG towards:
(1) renaming offset-inline-* and offset-block-* from the logical properties
ED, given that Firefox has shipped them
(2) sticking with the (imperfect) motion-* names from the motion path WD,
given that Chrome has shipped them
(3) choosing a new, non-conflicting name for motion-* RealSoonNow so that
we can switch to that instead.

Thoughts?

Thanks,
    -Shane