TEIIDDES-2120: Move Xsd files from models to other files section of vdb

* VdbHelper
 * Replace static strings with VdbFolder enum that also incorporates a file
   filter, read and write folders
 * getFolder() should be able to return the project as the 'root folder'
   given a suitable folderName, ie. ".". Does imply that the return object
   must be a container rather than a folder
 * Other Files enum only references otherFiles directory when creating and
   adding a file from the filesystem. When reading from the project it
   searches from the root of the project. Necessary since Xsd files can
   appear anywhere in the project.

* VdbFileDialogUtil
 * Passes into methods VdbFolders enum value instead of vague boolean flag

* plugin.xml
* VdbBuilder
* VdbDuplicateNamesMarkerResolutionGenerator
* VdbMigrateXsdFilesMarkerResolution
 * Provides a quick fix to migrate Xsd files from the vdb's models
   collection to its otherFiles collection

* VdbEditor
 * Refactor duplicate code into single addSelectionToVdb method

* Vdb
 * Provides API for migrating Xsd files from the models collection to the
   other files collection and deleting the index file

* VdbModelEntry
 * Broadens scope of imports collection to contain file entries as well as
   model entries
 * replaceImport method to replace import entry references

* vdb-deployer.xsd
 * Comments out the constraint that stops vdbs from containing duplicate
   named models. This logic is already implemented in code and designer
   must support old vdbs that may contain such duplicate models and give
   the user the chance to fix them.