RFR: JDK-7154070: in SwingSet2, switching between LaFs it's easy to lose JTable dividers

[Prasanta Sadhukhan]

In SwingSet2 in TableDemo, we can see grid dividers in Metal L&F but 
if we Switch to the Nimbus LaF which has no grid dividers by default and then if we switch to Java (Metal): it will show no dividers.

Issue is Nimbus call JTable.showGrid(false) in installDefaults but fail to reset in uninstallDefaults which is now rectified.

No regression test is added as it can be verified by SwingSet2 Tabledemo..

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Commit messages:
 - JDK-7154070: in SwingSet2, switching between LaFs it's easy to lose JTable dividers

Changes: https://git.openjdk.org/jdk/pull/12385/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12385&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7154070
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12385.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12385/head:pull/12385

PR: https://git.openjdk.org/jdk/pull/12385