Web Manager unified ID.
A WmId object is used to uniquely identify a persistent object in WM regardless of whether it is stored in
a JSR-170 compliant repository or as a row in an SQL table, or something else. Implementations of this
interface will typically encapsulate a ID specific to a given persistence mechanism, such as a primary key
for a SQL table, or a UUID in JCR.
Implementors of this interface must redefine the following methods:
Object.toString() - This must return a string identifier for his object. Implementations are
recommended to use a simple domain_prefix:domain_id scheme for building up strings. e.g. siteworks:1234,