[WSS4J 1.6] Specifying elements to sign or encrypt

The signature and encryption creation code in WSS4J uses the WSEncryptionPart class to find DOM elements to sign and encrypt. There are a number of minor changes to how elements are located from a WSEncryptionPart in WSS4J 1.6:

1. WSEncryptionPart now stores an optional DOM element, which will be used as the element to sign/encrypt if it is non-null.
2. Failing this, it finds the SOAP body and compares the wsu:Id with the stored Id, or if there is no stored Id in WSEncryptionPart, it checks the stored localname/namespace.
3. Failing this, if the stored Id in WSEncryptionPart is not null, it tries to find the first element in the SOAP envelope that has a matching wsu:Id.
4. If the stored Id is null, it tries to find *all* DOM Elements that match the stored localname/namespace.

WSEncryptionPart is intended to refer to a single Element for encryption/signature. However, as a localname/namespace is not necessarily unique, point 4 will return all matching Elements. An important implication of the order of the steps given above, is that client code should set the DOM element on the WSEncryptionPart if it is accessible, and if not, it should set the wsu:Id. Otherwise, a localname/namespace (which is not referring to the SOAP Body) will result in a traversal of the DOM tree.

The DOM element(s) that is(are) found are stored for retrieval, so that we don't need to traverse the SOAP envelope multiple times, when e.g. doing an STR Transform, or for element location in the XML Security code.