RFR: 8277133: Dragboard contents retrieved all over again during a DND process on WebView

[Jose Pereda]

This PR improves drag and drop events on a WebView node, by generating the content from the dragboard only when the drag events starts, and not for every drag over event.

A manual test is included that shows how the drag and drop experience improves, especially when dragging online images, as the time between each consecutive drag over event drops from hundreds of ms to tens of ms or even lower.

This PR is somehow blocked by #668, as dragging some images fails on macOS.

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Commit messages:
 - Add manual test
 - Generate mime/values only when drag event starts

Changes: https://git.openjdk.java.net/jfx/pull/680/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=680&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277133
  Stats: 99 lines in 5 files changed: 89 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/680.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/680/head:pull/680

PR: https://git.openjdk.java.net/jfx/pull/680