RFR: 8279228 Leak in ScrollPaneSkin, related to touch events

[Florian Kirmaier]

Fixing memoryleak, related to touch events in ScrollPaneWhen touchDetected or mouseDown is true, the sbTouch animation is running, 
and the node is removed from the Scene, then the animation will never stop, causing a memory leak.
A simple fix is to also check, whether the Node is visible, by checking the "isTreeShowing" property.

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Commit messages:
 - JDK-8279228

Changes: https://git.openjdk.java.net/jfx/pull/701/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=701&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279228
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/701.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/701/head:pull/701

PR: https://git.openjdk.java.net/jfx/pull/701