Changed Jump-To-Definition, as requested -- modeled on QuickEdit:

- Now can register multiple jump-to-definition providers (only one, so far).
  - Moved jump-to-definition menu setup to top level.
  - Added jump-to-definition to floating right-click menu. [ Note: we may not want
    this unless there is a provider registered for the current mode, but that's how
    QuickEdit does it.  We can fix both at the same time if we decide to. Also note
    that if we want to change the right-click menu for this, one could argue that we
    should also change the top level menu for the same reason. ]