RFR: 8246745: ListCell/Skin: misbehavior on switching skin

[Jeanette Winzenburg]

ListCellSkin installs listeners to the ListView/fixedCellSize that introduce a memory leak, NPE on replacing the
listView and incorrect update of internal state (see bug report for details)

Fixed by removing the listeners (and the internal state had been copied from listView on change) and access of listView
state when needed.

Added tests that failed before and pass after the fix, plus a sanity test to guarantee same (correct) behavior
before/after.

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

Commit messages:
 - 8246745: ListCell/Skin: misbehavior on switching skin

Changes: https://git.openjdk.java.net/jfx/pull/251/files
 Webrev: https://webrevs.openjdk.java.net/jfx/251/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246745
  Stats: 98 lines in 4 files changed: 59 ins; 33 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/251.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/251/head:pull/251

PR: https://git.openjdk.java.net/jfx/pull/251