RFR: 7004: ThreadView : arrow up/down doesn't work

[Alex Macdonald]

This PR address JMC-7004 [[0]](https://bugs.openjdk.org/browse/JMC-7004), in which the up + down arrow keys don't do anything on the threads page "table". The table on the threads page is actually a split version of the `ChartCanvas`, so any table-like behaviour is/was added into it. In this case, the `ChartTextCanvas` will now pass the current lane height to the `XYChart` which will use the value to select the next thread lane up or down depending on the keyboard action. 

[0] https://bugs.openjdk.org/browse/JMC-7004

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Commit messages:
 - 7004: ThreadView : arrow up/down doesn't work

Changes: https://git.openjdk.org/jmc/pull/492/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=492&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7004
  Stats: 36 lines in 2 files changed: 35 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmc/pull/492.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/492/head:pull/492

PR: https://git.openjdk.org/jmc/pull/492