* vtkSMPluginManager now works with multiple sessions. Moved vtkSMPluginManager
  from vtkSMSession to vtkSMProxyManager. vtkSMPluginManager keeps track of
  plugins loaded on local processes and remote processes for each of the
  sessions correctly.
* pqPluginManager now works with multiple sessions. Changed API to require the
  user to specify the session without assuming an active session.
* Fixed Python modules following vtkSMPluginManager/vtkSMProxyManager API
  changes.