RFR: 8262518: SwingNode.setContent does not close previous content, resulting in memory leak

[Prasanta Sadhukhan]

Issue is when setting the content of a SwingNode, the old content is not garbage collected owing to the fact
JLightweightFrame is never being released by SwingNodeDisposer

The SwingNodeDisposer holds an hard pointer to the JLightweightFrame that prevents its collection

Modified `SwingNode.setContentImpl`  function to use a WeakReference to properly release the memory.

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

Commit messages:
 - 8262518: SwingNode.setContent does not close previous content, resulting in memory leak
 - 8262518: SwingNode.setContent does not close previous content, resulting in memory leak

Changes: https://git.openjdk.org/jfx/pull/1219/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1219&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8262518
  Stats: 14 lines in 2 files changed: 5 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1219.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1219/head:pull/1219

PR: https://git.openjdk.org/jfx/pull/1219