<Swing Dev> RFR: 8264950: Set opaque for JTooltip in config file of NimbusLookAndFeel

[Tejpal Rebari]

Set the opaque property for JToolTip in config file (skin.laf) of NimbusLookAndFeel instead of setting up in the SynthToolTipUI.
This is related to the discussion on PR  https://github.com/openjdk/jdk/pull/3167.

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Commit messages:
 - set opaque for JTooltip in config file

Changes: https://git.openjdk.java.net/jdk/pull/3837/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3837&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264950
  Stats: 3 lines in 3 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3837.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3837/head:pull/3837

PR: https://git.openjdk.java.net/jdk/pull/3837