RFR: 7091: JOverflow Instance View table should be sortable by clicking headers

[Alex Macdonald]

This PR addresses JMC-7091 [[0]](https://bugs.openjdk.java.net/browse/JMC-7091), in which the tree viewer in the JOverflow instances tab does not allow the table to be sorted via the column headers. Currently the viewer column headers do not perform any action when clicked. Now a comparator have been added to the viewer which sorts based on the column header (String comparison for the Name & Value columns, Integer for the Size column).

Screenshot:
![2021-10-14-142941_1074x516_scrot](https://user-images.githubusercontent.com/10425301/137375980-daa9e849-5aec-41c8-9cb5-00104a15e682.png)

Gif:
![7091](https://user-images.githubusercontent.com/10425301/137375958-a9186bf8-36ea-43e3-91b5-ee85aaca9037.gif)

[0] https://bugs.openjdk.java.net/browse/JMC-7091

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Commit messages:
 - 7091: JOverflow Instance View table should be sortable by clicking headers

Changes: https://git.openjdk.java.net/jmc/pull/322/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=322&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7091
  Stats: 60 lines in 1 file changed: 54 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jmc/pull/322.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/322/head:pull/322

PR: https://git.openjdk.java.net/jmc/pull/322