T: switch UE in enb tracer

Just below the logo you see the current UE, eg. [UE 0],
and near that you have two "buttons" [prev UE] and [next UE].

You click with left button of mouse on those and the various
plots that depend on the UE are now done for the new UE (say UE 1
instead of UE 0 if you click on [next UE]).

The plots are not cleared when you switch.
Let's say it's a feature.