Update check for spacelike mml nodes to be more sensible. (mathjax/MathJax#3098)

[dpvc]

This PR adjusts the way that mspace and mtext nodes are treated as spacelike. The spec says they are always spacelike, but that causes all kinds of problems, like making {\text{a}=\text{b}} being treated as an embellished operator. This interferes with line-breaking, for example, since a break at the equal sign will be moved to outside the text elements, which is not what is usually wanted. Similarly,

<math>
  <mtext>a</mtext>
  <mspace linebreak="newline">
  <mo>=</mo>
  <mtext>b</mtex>
</math>

is also an embellished operator, and so treated as a single unit.

This PR makes mtext be spacelike only if it consists only of spaces, and doesn't have styles or background colors. Similarly, mspace will not be spacelike if it has an explicit linebreak attribute (or if it is not breakable due to other attributes). This is not according to the spec, but makes more sense, particularly for how we use mtext and mspace in translating TeX to MathML.

Resolves part of issue mathjax/MathJax#3098.

[zorkow]

One comment.
O/w fine.

[zorkow]

Should those attributes go into a dedicated constant, in case we need more or need to change them in the future?

[dpvc]

There are a couple of places where this type of check is being made, and I was thinking about making a service function to check them. I'll look into that.

[dpvc]

I have added hasOneOf() to the Attributes object to handle this, and made lists for the styles to use for space and mtext elements. I also added a hasExplicit() method to the Attributes object to make checking for explicit attributes more natural, and unset() to make removing an attribute easier. I edited a number of other files to use these new functions. So there are a bunch of new edits here. Do you want to review again?

[zorkow]

lgtm.