Modify Vdb class to accomodate vdb imports element

To re-enable our merge VDB concept for Preview data, we'll need a way to add VDB imports to the project preview VDB's "vdb.xml". The easiest way would be to add the JAXB support to the Vdb class for <import-vdb> element and it's sub-structure.

Lastly we need to add addImportVdb() and removeImportVdb() methods to the Vdb class.

In particular, these methods need to be made available to the PreviewManager and the corresponding deploy/merge VDB utilities.