Guarding diagrams against server version change

* Should the server version be changed, diagrams that are open or already
  have been opened will have elements that have been cached. Such caches
  cause exceptions since they contains models comprising of runtime specific
  language objects, which are not common between runtime clients. Thus,
  can cause class cast exceptions in the query service

* ModelerCore
 * Ensure editors are closed upon a change of server version

* DiagramEntityManager
 * Clear the diagram cache on change of version

* SqlMappingRootCache
 * Clear the mapping caches on change of version

* DqpPlugin
* PreviewModelObjectLabelProvider
 * Initialise the default server version as early as possible to avoid an
   unnecessary server version change