Improve usability of set default action

* ModelerCore
 * Only close editors if the server version actually changes, as different
   servers but the same version will not require a different runtime client

* SetDefaultServerAction
 * Added to the Guides view so taiored accordingly
 * If their is a change in server version and there are open editors then
   ask the user if they wish to continue since the editors will be closed

* ServerSelectionDialog
 * Cosmetic improvements to layout
 * Better map for storing keys to servers

* RuntimeAssistant
 * Used for executing the default server action in the same manner as the
   edit server action
 * selectServer method for displaying the ServerSelectionDialog, logic taken
   from the EditServerAction and generalised.