RFR: 7031: Build a constant pool explorer page

[Jean-Philippe Bempel]

Add under Recording a new Page for ConstantPools
Help to visualize what's inside constant pools part of a JFR recording

![JMC_ConstantPoolsPage](https://user-images.githubusercontent.com/4610701/110855759-13569700-82b7-11eb-9981-deb0b8b5e185.png)

*Draft*
- Icon is copy/paste from recording. If you have idea about how to replace it...
- I have limited the tables to the search toolbar, visible by default
- a hidden count column for constant values helps you find out when you duplicates constant (for OldObjectSample event issue) should not be useful otherwise

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Commit messages:
 - Constant pool icon
 - fix bug check
 - fix bug check
 - 7031: Build a constant pool explorer page

Changes: https://git.openjdk.java.net/jmc/pull/228/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=228&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7031
  Stats: 522 lines in 20 files changed: 507 ins; 1 del; 14 mod
  Patch: https://git.openjdk.java.net/jmc/pull/228.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/228/head:pull/228

PR: https://git.openjdk.java.net/jmc/pull/228