Some cleanups to the SC 1.3.1 label code (#107).

Now correctly emits a warning (H65 or ARIA6) when the title or
aria-label attribute is specified but empty.

Hidden field WARNING now has a sub code F68.Hidden.

The final "this field is not labelled" is now an ERROR, not a WARNING.

A test file has been written for this (1_3_1_Labels.html), currently only handling text fields (and
fields without an input type, defaulting to text) but covering explicit
labels, wrapped labels, title attr, aria-label, and aria-labelledby.