[FIX] im_livechat: various fixes

- Don't display livechat button if no operator available
- Livechat windows persistence through website pages
- Auto popup feature re-introduced (inadvertently removed at rev. f23f57cc)
- Operators that are 'away' are available, as 'away' means that the focus isn't
  on the Odoo tab for at least 10 minutes, but the user is still connected.
- Avoid duplicated messages when the visitor is a connected user
- Unfold livechat window on first 'close' click as the event is intercepted to
  ask the user feedback before actually closing the window