Abtract away the element/attribute structure

[fred-wang]

In general, the structure handled by TeXZilla is a string representing an element.

The serialization could be defered and we could instead handle a structure containing:

• an element name
• a list of (attribute, value) pairs on that element.
• a string representing the children

This will allow to:

• avoid duplicate attributes (not well-forme XML). As in itex2MML, this may happen with table attributes.
• avoid to use useless mstyle or mrow. For example the mathvariant or color attributes can just be set on token elements.

[fred-wang]

* avoid duplicate attributes (not well-forme XML). As in itex2MML, this may happen with table attributes.

I committed a first refactoring that should get rid of duplicate attributes in tables. For example, that used to be the case for \array{ \arrayopts{\collayout{left right}\colalign{right left}} a & b} (unit test added). Reading the code, I didn't find any other case when that can happen.