Widget annotation: implement field name according to the specification

The original code is difficult to read and, more importantly, performs
actions that are not described in the specification. It replaces empty
names with a backtick and an index, but this behavior is not described
in the specification. While the specification is not entirely clear
about what should happen in this case, it does specify that the `T`
field is optional and that multiple field dictionaries may have the same
fully qualified name, so to achieve this it makes the most sense to
ignore missing `T` fields during construction of the field name. This is
the most specification-compliant solution and, judging by opened issue #6623, also the required and expected behavior.

examples/acroforms/forms.js

@@ -91,7 +91,7 @@ function setupForm(div, content, viewport) {
             // select box is not supported
           }
           input.className = 'inputControl';
-          input.name = item.fullName;
+          input.name = item.fieldName;
           input.title = item.alternativeText;
           assignFontStyle(input, item);
           bindInputItem(input, item);