TEIIDES-2094: Introduce viewing, saving and loading of teiid imported meds

* TeiidInstanceMedListener
 * When acquiring a med from teiid instance, mark it as imported

* ModelExtensionRegistryView
 * Ensures only 1 registry listener is added to med registry and that it
   is removed upon disposal of view
 * Displays an 'Imported' column to show which meds have been imported
 * Workaround an eclipse bug when checkbox images are not centred in swt
   table cells
 * When viewing a med:
  * copy a built-in med to the built-in project as before
  * create a link to an imported med once its saved to its actual location
    in the runtime plugin metadata directory
 * When receiving changes from the registry ensures that refreshes to the
   view are made on the UI thread

* ExtensionPlugin
 * Load and save both the user-defined and imported med definitions located
   in the runtime plugin metadata directory

* ExtensionDefinitionsManager
 * Renames UserExtensionDefinitionsManager as it now covers the imported
   definitions as well