GUI: support for observer role

 - allow to display vos/members tab
 - vo and members are clickable for new role
 - added support in session

TODO: groups tabs, content of all tabs, where vo_admin or vo_observer.
GUI: more support for VoObserver role

- VoObserver can now browse all tabs like vo admin, but all buttons
  and table checkboxes should be disabled.
- Tabs for add/assign/edit are not accessible for new role.
- Added new method to JsonCallbackEvents which can be used to pass data
  (results) from one callback to another.
- Added some custom messages for empty tables to vo/group management.
- Code format and cleanup.
GUI: more custom texts on empty tables

- Added more custom texts displayed on empty tables.
- Source format.