RFR: 5021949: JSplitPane setEnabled(false) shouldn't be partially functional

[Prasanta Sadhukhan]

Issue is seen in that if we call setEnabled(false) over JSplitPane than it can't be dragged via its divider, But if SplitPane have one touch expandable true than user can click those buttons and change the divider position. 
So, if splitpane is disabled, then both dragging in divider and one-touch-expandable click should be disabled.
Fix is made to override setEnabled and disable one-touch-expandable buttons actions..

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Commit messages:
 - 5021949: JSplitPane setEnabled(false) shouldn't be partially functional

Changes: https://git.openjdk.org/jdk/pull/19695/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19695&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-5021949
  Stats: 156 lines in 3 files changed: 153 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19695.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19695/head:pull/19695

PR: https://git.openjdk.org/jdk/pull/19695