TEIIDDES-1775: Centralise the handling of the default server preference

* Rather than the ModelExplorerResourceNavigator listening for the server
  preference change, have the TeiidServerManager deal with it in the same
  manner as a default server change. Ensures that editors can be closed
  if necessary.

* DefaultTeiidServerPreferenceContributor
 * Since editors may be closed as a consequence of the server version
   preference being modified, ask the user if this is acceptable and if
   not then do not apply the change in preference.
 * Sort the runtime version available with the most recent at the top.

* TeiidServerManager
 * Adds the preference listener
 * Run the closeEditors function on the UI thread to avoid any possibility
   of invalid thread access exceptions
Merge pull request #206 from phantomjinx/TEIIDDES-1775

TEIIDDES-1775: Centralise the handling of the default server preference