fix(layout): allow layout-align without cross-axis value

[ngraef]

• fix parsing issue to allow single value
• add demo to test omitting second value

fixes #5996

[ThomasBurleson]

@ngraef - this is not ideal. Allowing a mainAxis-only is fine, but the style should always fallback to defaults:

• main-axis === 'start'
• cross-axis === 'stretch'

If you update the demos accordingly, I will merge.

Meanwhile, this is already in place in the generation-2 Layout engine coming in January.

[ngraef]

@ThomasBurleson I'm confused about what needs work. This doesn't change anything about the default values or the fallback behavior during parsing.

[ThomasBurleson]

@ngraef - This PR is incomplete:

• Layout.js needs updated with layout-align improvements for fallbacks/defaults
• $scope.layoutAlign should not be checking for a blank crossAxis (that should be done in fallbacks).

I will curl your PR and fix.

[ngraef]

@ThomasBurleson

• layout.js already has a fallback to default values in extractAlignAxis:

  if ( ALIGNMENT_MAIN_AXIS.indexOf(axis.main) < 0 )   axis.main = "start";
  if ( ALIGNMENT_CROSS_AXIS.indexOf(axis.cross) < 0 ) axis.cross = "stretch";

  I'm not sure what other improvements you want.

• My change to $scope.layoutAlign simply omits the trailing whitespace if the new none (use default) option is selected in the demo. I suppose a trim might be better:

  return ($scope.layoutDemo.mainAxis + ' ' + $scope.layoutDemo.crossAxis).trim();

  This was done mostly for aesthetic reasons, because the demo would previously display layout-align="space-around ". However, it also helped me test my changes to layout.js, because layout-align="space-around " and layout-align="space-around" were being parsed differently, which is exactly the bug this PR fixes.

[ThomasBurleson]

@ngraef - I think my current Flu is confusing me; since I wrote

    if ( ALIGNMENT_MAIN_AXIS.indexOf(axis.main) < 0 )   axis.main = "start";
    if ( ALIGNMENT_CROSS_AXIS.indexOf(axis.cross) < 0 ) axis.cross = "stretch";

LOL.

Thank you for the clear and reasonable follow-up.
This looks good and I will merge soon.

[ngraef]

@ThomasBurleson - No problem. Thanks for reviewing.