Bring back GC summary screen providing all the information which was present
on GC times screen.

We need this in place so that:
a) GC overhead can be computed easily (given Total GC Time and Recording
Time)
b) Overall GC Statistics readily available
c) Summary of Young GC metrics (count, avg./max/total times) readilyavailable
d) Summary of Old GC metrics (count, avg./max/total times) readily available