[GUI] changed logic of follow mode toolbar button

Clicking on the button will now switch follow mode to the next
session user. Furthermore a dropdown menu has been added
to select a specific user who should be followed.

Change-Id: Id4eb00a5ee3519bb111c1393364efb2f09ac4dc7
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/890
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>