RFR: 6922: Reintroduce GC summary screen

[Suchita Chaturvedi]

This PR addresses the enhancement of adding a new screen "GC Summary" which will cover missing information from "GC Times" screen present in JMC 5.5.

Here is the preview of the new screen added for the reference:

![image](https://user-images.githubusercontent.com/11155712/125062925-8a1c6900-e0cc-11eb-992e-a40a72a21ffe.png)

Here is the preview of the reference screen from JMC 5.5

![image](https://user-images.githubusercontent.com/11155712/125063237-dc5d8a00-e0cc-11eb-86d8-37a883511d45.png)

I have opened same JFR in both the versions, so that values can be compared while review.

Please review the changes and let me know your valuable comments.

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Commit messages:
 - 6922: Reintroduce GC summary screen

Changes: https://git.openjdk.java.net/jmc/pull/275/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=275&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6922
  Stats: 278 lines in 10 files changed: 277 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/275.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/275/head:pull/275

PR: https://git.openjdk.java.net/jmc/pull/275