RFR: 8242621: TabPane: Memory leak when switching skin [v4]

[Ambarish Rapte]

> `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.

Ambarish Rapte has updated the pull request incrementally with two additional commits since the last revision:

 - Space corrections in tests
 - Review update: cleanup and corrections

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/318/files
  - new: https://git.openjdk.java.net/jfx/pull/318/files/a3b57950..93e1645f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=318&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=318&range=02-03

  Stats: 63 lines in 3 files changed: 0 ins; 15 del; 48 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