* src/application-events.cpp (ApplicationEvents::Invoke): Invalidate
  UI when a new Item is loaded.
* src/gpgoladdin.cpp (GpgolRibbonExtender::Invoke): Store RibbonUI
  instances. Forward calls for new ID's.
  (GpgolRibbonExtender::GetCustomUI): Add callbacks.
  (GpgolRibbonExtender::GetIDsOfNames): Add CB ID's-
  (gpgoladdin_invalidate_ui): Invalidate UI. Invalidate causes
  all callback to be triggered again in their correct contexts.
* src/gpgoladdin.h: Add prototype for gpgoladdin_invalidate_ui.
* src/ribbon-callbacks.cpp (mark_mime_action): Work on base message.
  (get_crypt_pressed): Callback for toggle buttons.
* src/ribbon-callbacks.h: Add ids and prototypes