RFR: 8334900: IOOBE when adding data to a Series of a BarChart that already contains data

[Markus Mack]

This PR is a fix for another IOOBE that I discovered while working on #1476.

The PR simplifies the code for adding a series that already contains data by adding the data points one-by-one.
As far as I can see no attempt was previously made to optimize the bulk operation except for some trivial O(1) operations, so this should have no noticable performance impact.

Accidentally this fixes another bug related to the missing "negative" style class when negative data values are added.

Also, the PR aligns the handling of duplicate categories with the behavior clarified in #1476, when there are duplicates in the data that was already in the series before the series was added to the chart.

Note a change was made to the createTestSeries() test method, letting it start at index 1, avoiding the duplicate data items resulting from multiplying by 0.
Without this change `testSeriesRemoveAnimatedStyleClasses` would fail because it counts the number of plot children, where the duplicates are now removed.

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Commit messages:
 - BarChart: Fix adding non-empty series
 - BarChart: Add styleClass "negative" for added data

Changes: https://git.openjdk.org/jfx/pull/1488/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1488&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334900
  Stats: 82 lines in 2 files changed: 59 ins; 21 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1488.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1488/head:pull/1488

PR: https://git.openjdk.org/jfx/pull/1488