Add a small default server status panel to Model Explorer view

* Provides a small collapsible panel that shows the current default teiid
  server and version

* ModelerCore
 * Need to allow server manager to set the default server to null if all
   servers are deleted
 * Notify listeners of server change even if there is not a server version
   change

* ITeiidServerVersionListener
 * Addition of method for serverChanged so listeners can be notified of
   a change of server even without a change of server version

* ModelExporerResourceNavigator
 * Adds form section to bottom of view, showing current default server and
   its version.
 * Hyperlinks to servers view from value of server
 * Updates on server and server version changes
Merge pull request #98 from phantomjinx/master

Panel for default server