GUI: added support for groups as entities managers

 - TabMenu - proper way of styling and widget putting
 - added calls to get/add/remove managers groups
 - added new buttons translation
 - groups can be VO/Group/Facility managers
 - removed unnecessary code
 - groups to be added as managers are offered based on VO/Group admin rights
 - if 'members' group is about to be added, warning is show to inform user about consequences