Fix problem with Libhover saving libraries at end of session

- the Libhover class has a saveLibraries() method that it is static
  which is called at workspace shut-down and it needs the state
  location path and preferenceStore which it tries to get from the
  LibhoverPlugin
- since the LibhoverPlugin stop() method nullifies the singleton,
  we might be trying to use a null plug-in value to acquire the
  preferenceStore and state location path from and this results
  in an NPE logged
- instead put such data into statics to use at shut-down and negate
  the need for the LibhoverPlugin singleton

Change-Id: I5831ccc2f16d99d9ccbedd0e254a08d2248ae864
Reviewed-on: https://git.eclipse.org/r/35912
Tested-by: Hudson CI
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>
Tested-by: Jeff Johnston <jjohnstn@redhat.com>