Here we convert the logical padding properties into their new resolved-at-
cascade-time implementations. This involves:
* converting -moz-padding-{start,end} into logical longhand properties
* adding padding-inline-{start,end} aliases for -moz-padding-{start,end}
* converting padding-{left,right} into longhand properties
* removing padding-{left,right}-value and
padding-{left,right}-{ltr,rtl}-source internal properties
The CSS parser and various tests are simplified a bit as a result.
