jackrabbit-docviewparser: optionally accept undeclared prefixes in file name

Although Enhanced DocView Format mandates namespace declaration of the prefix used in the filename, this often conflicts with automatic cleanup of XML editors if the prefix is not also used within the xml itself.
During import both the already declared prefixes from the target repository and the declared prefixes from the XML are considered, so in most cases it should work without declaration anyways.

Therefore the validator should be relaxed to optionally not emit errors for undeclared prefixes in the file name of docview files.