TEIIDDES-2120: Addresses shortcomings of adding imported xsd files

* Closes loopholes with addition of xsd files to a Vdb in the VdbEditor

* VdbUtil
 * canAddModelToVdb checks for duplicate models already in Vdb. However,
   the model resource references includes Xsd files which although are
   resources are included in the 'Other Files' section and a model can
   have the same name as an Xsd file.

* Vdb
 * Avoids further loopholes by making a single addEntry method which both
   model and file entries are added through. Consequently, only one
   function path to determine how the given resource should be added.

* NewVdbWizard
 * Stop Xsd files from being displayed in the Add Models chooser dialog
TEIIDDES-2120: Fixes failing unit tests

* VdbEntry
 * Should be abstract since only ModelEntry and FileEntry are applicable
   to Vdbs

* VdbFileEntry
 * Fixes possibility of the entry name being null in circumstances where
   the FileEntryType could be null. Only cropped up in unit tests

* VdbEntryTest -> VdbFileEntryTest
 * Crazy to be testing the abstract VdbEntry being added to a Vdb when no
   such entries are ever added.
 * Use of VdbFileEntries has meant slight differences in how the tests
   are implemented.