[CSSWG][css-align][css-grid] Generalization of Grid's 'gap' properties: Update to CSS Box Alignment Level 3 (late announce)

Probably the worst announcement to have missed this year...

In August the CSSWG resolved to drop the 'grid-' prefixes of Grid Layout's gutter
properties, 'grid-gap'/'grid-row-gap'/'grid-column-gap', merging its row gap with
the existing multicolumn module's 'row-gap' property and extending its functionality
to apply to Flexbox as well. See https://github.com/w3c/csswg-drafts/issues/1696

As a result, on 6 September 2017 the CSS Working Group published an updated
Working Draft of the CSS Box Alignment Module Level 3, shifting the definitions
of these properties (and renaming them accordingly) to this module:
   https://www.w3.org/TR/css-align-3/#gaps
(The Grid module has also been updated to remove the grid-gap definitions.)

There were no other changes since the draft six weeks prior:
   https://www.w3.org/TR/2017/WD-css-align-3-20170906/#changes

As always, please review the draft, and send any comments to this mailing list,
<www-style@w3.org>, prefixed with [css-align] (as I did on this message) or
(preferably) file them in the GitHub repository at
   https://github.com/w3c/csswg-drafts/issues

For the CSS WG,
~fantasai