fix caret timeline memory leak

[johanvos]

Stop timeline (and remove it from the GC root) when the ParagraphTile is removed from the scenegraph

[eugener]

Minor observation:
caretTimeline.play() in the newScene != null branch runs unconditionally, meaning all layers (not just the one with the caret) briefly start their timeline on scene attachment. This is harmless in practice because setCaretVisibility gates on !caretShape.getElements().isEmpty() (making the animation frames no-ops), and updateCaretPosition() will quickly stop the timeline for non-caret layers. A slightly more precise version would
guard with if (hasCaret()) before playing, but it's not worth complicating the fix for.

[johanvos]

Minor observation: caretTimeline.play() in the newScene != null branch runs unconditionally, meaning all layers (not just the one with the caret) briefly start their timeline on scene attachment. This is harmless in practice because setCaretVisibility gates on !caretShape.getElements().isEmpty() (making the animation frames no-ops), and updateCaretPosition() will quickly stop the timeline for non-caret layers. A slightly more precise version would guard with if (hasCaret()) before playing, but it's not worth complicating the fix for.

Good remark.
I added the hasCaret() check. It is a non-intrusive say check, and I believe it's the right thing to do. I'm not sure about the cost (including object creation) of starting a timeline (even without actions). Since this code runs on the FX thread, every single instruction counts.