RFR: 8279214: Memory leak in Scene after dragging a cell

[Marius Hanl]

This PR fixes a memory leak in dnd code in the `MouseHandler` of a `Scene`.
The memory leak occurs after calling `startFullDrag()`. The `fullPDRTmpTargetWrapper` is then populated but never cleared.
Fix is to call `clear()` on the `fullPDRTmpTargetWrapper`, very similar to https://github.com/openjdk/jfx/pull/448 ([JDK-8264330](https://bugs.openjdk.java.net/browse/JDK-8264330)).

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Commit messages:
 - 8279214: Memory leak in Scene after dragging a cell

Changes: https://git.openjdk.org/jfx/pull/899/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=899&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8279214
  Stats: 42 lines in 2 files changed: 42 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/899.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/899/head:pull/899

PR: https://git.openjdk.org/jfx/pull/899