RFR: 7175397: The divider color is not changed to green when dragging for Nimbus LaF.

[Prasanta Sadhukhan]

SplitPaneDivider.draggingColor UIProperty was not honoured in Nimbus L&F. 
Added support for setting SplitPane dragging color for Nimbus L&F by drawing a fillRect of the set color as done in BasicLookAndFeel.
The fix relies on `continuousLayout` property being false by default which is enabled for Nimbus, which is set to default false now, same as in other L&F. I think it was set true just to have documented default value in JDK-6937415

All jtreg swing tests are ok along with SwingSet2 JSplitPane demo in Nimbus L&F.

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Commit messages:
 - 7175397: The divider color is not changed to green when dragging for Nimbus LaF.

Changes: https://git.openjdk.org/jdk/pull/9937/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9937&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7175397
  Stats: 26 lines in 2 files changed: 25 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9937.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9937/head:pull/9937

PR: https://git.openjdk.org/jdk/pull/9937