<Swing Dev> RFR: 8240709: Enable javax/swing/UI/UnninstallUIMemoryLeaks/UnninstallUIMemoryLeaks.java on all L&F

[Sergey Bylokhov]

Fix for various memory leaks in Nimbus and Motif L&Fs, see comments inline.

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Commit messages:
 - initial fix

Changes: https://git.openjdk.java.net/jdk/pull/595/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=595&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240709
  Stats: 111 lines in 4 files changed: 67 ins; 18 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/595.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/595/head:pull/595

PR: https://git.openjdk.java.net/jdk/pull/595