RFR: 5721: Reintroduce the Percentage column

[Dmitry Popov]

This patch addresses [JMC-5721](https://bugs.openjdk.java.net/browse/JMC-5721): Reintroduce the Percentage column.

1. StacktraceView has been amended to display Percentage column as well it displays stack trace and count ones. Percentage format is #.## (up-to 2 decimal points), it will also arbitrarily round the number.
2. Tool tip with frame fraction and sibling groups info has been moved from Count column to Percentage.
3. Currently Background drawer decorates Percentage column instead of Count.

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Commits:
 - b3ad0225: JMC-5721 :: StacktraceView :: Reintroduce the Percentage column

Changes: https://git.openjdk.java.net/jmc/pull/12/files
 Webrev: https://webrevs.openjdk.java.net/jmc/12/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-5721
  Stats: 23 lines in 5 files changed: 16 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jmc/pull/12.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/12/head:pull/12

PR: https://git.openjdk.java.net/jmc/pull/12