RFR: 8209017: CheckBoxTreeCell: graphic on TreeItem not always showing

[Nir Lisker]

A simple fix for the graphics of a `CheckBoxTreeCell` not showing after expand/collapse events. The graphics node needs to be released from the previous checkbox so that it doesn't serve as the graphics of more than one checkbox at once. If it does, only one checkbox (its skin) can actually use it as a scenegraph child, and it's not always the correct one.

Added also a manual test app as I'm unsure about how to do an automatic test for this. The test app will be useful for later fixes, so I suggest adding it anyway. To test the fix, simply expand and collapse the root note repeatedly while it has a child. Before the fix the graphic will disappear after a few tries, after the fix it won't. You can test other hierarchies as well by adding children to the tree.

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Commit messages:
 - Removed whitespaces
 - Name and whitespace fixes to the test app
 - Manual test utility
 - Fix

Changes: https://git.openjdk.org/jfx/pull/970/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=970&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8209017
  Stats: 112 lines in 5 files changed: 111 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/970.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/970/head:pull/970

PR: https://git.openjdk.org/jfx/pull/970