Set the id of the resource containing the access key for this label.
It may be null. In this case the default resource id is used:
<label-id>.access-key
(where label-id is the resource id containing the label for the element.
if it exists).
If such resource exists and specifies a single character, the
tag outputs an additional accesskey attribute for the HTML a <label> element.
If given resource does not exist, it's also ok. Noone will die.