RFR: 8290990: Clear .root style class from a root node that is removed from a Scene/SubScene

[Michael Strauß]

When a node is set as the root node of a `Scene` or `SubScene`, the "root" style class is automatically added to the node, but not cleared when the node is later removed from the scene.

This can lead to an incorrectly set "root" style class when the removed node is inserted in the scene graph below the root node, or to duplicate "root" style class entries when the removed node is again set as the root node of the scene.

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

Commit messages:
 - Clear .root style class when root node is removed
 - Failing test

Changes: https://git.openjdk.org/jfx/pull/849/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=849&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290990
  Stats: 27 lines in 4 files changed: 25 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/849.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/849/head:pull/849

PR: https://git.openjdk.org/jfx/pull/849