RFR: 8242621: TabPane: Memory leak when switching skin

Tue Oct 13 13:00:15 UTC 2020

`TabPaneSkin` installs some listeners that are not removed when `TabPaneSkin` is changed.
The fix converts listeners to WeakListeners and also removes them on dispose.

There is a NPE check change needed in `isHosrizontal()`. Without this check it causes NPE if pulse is in progress while
TabPaneSkin is getting disposed.

`SkinMemoryLeakTest` already had a test which only needed to be enabled.
Test fails before and passes after this change.

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Commit messages:
 - 8242621: TabPane: Memory leak when switching skin

Changes: https://git.openjdk.java.net/jfx/pull/318/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=318&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242621
  Stats: 61 lines in 2 files changed: 43 ins; 10 del; 8 mod
  Patch: https://git.openjdk.java.net/jfx/pull/318.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/318/head:pull/318

PR: https://git.openjdk.java.net/jfx/pull/318