Content Model fidelity improvement 4: CENTER tag and alignment

[JiuqingSong]

I'll make a series changes to improve content model fidelity.

With HTML align attribute and CSS style text-align, there are complex combinations.

Say a sample HTML:

<div align={align} style="text-align: {textAlign}">
 <div>{text}</div>
 <table>{table}</table>
</div>

The combinations are:

text align	html align	text result	table result
NO	NO	No alignment	No alignment
YES	NO	follow text align	No alignment
NO	YES	Inherat html align	apply html align
YES	YES	follow text align	No alignment

For table, we handle alignment using its margin style. For other blocks, we use align style.

For CENTER tag, for now the easiest way is to use ContentModelGeneralBlock to just directly copy the HTML element itself.