correlate event metrics

[milianw]

cf. the original request from https://github.com/KDAB/hotspot/issues/14:

when collecting multiple perf event sources in one profile, Hotspot should correlate different metrics. For example we should:

• display CPI (cycles per instructions) when both cycles and instructions are measured in a group
• percentage of branch-misses, when we measure both branches and brach-misses
• dito for cache-misses
• dito for topdown metrics
• ...