RFR: 8296387: [Tooltip, CSS] -fx-show-delay is only applied to the first tooltip that is shown before it is displayed [v10]

[Marius Hanl]

> This PR fixes a long standing issue where the `Tooltip` will always wait one second until it appears the very first time, even if the 
> `-fx-show-delay` was set to another value.
> 
> The culprit is, that the `cssForced` flag is not inside `Tooltip`, but inside the `TooltipBehaviour`. So the very first `Tooltip` gets processed correctly, but after no `Tooltip` will be processed by CSS before showing, resulting in the set `-fx-show-delay` to not be applied immediately.
> 
> Added a bunch of headful and headless tests for the behaviour since there were none before.

Marius Hanl has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:

 - Merge branch 'master' of https://github.com/openjdk/jfx into 8296387-tooltip-css
 - Use Base64 encoder in tests
 - Improve time measurement and simplify test diff code
 - add many more unit tests for Tooltip
 - Use Helper class instead
 - Doc
 - Add a test for changing the stylesheet and always process CSS for that matter
 - Add more documentation and improve css stylesheet test threshold
 - Implement applyStylesheetFromOwner(..) and use it instead to ensure correct CSS processing for the Tooltip Node.
 - Merge branch 'master' of https://github.com/openjdk/jfx into 8296387-tooltip-css
 - ... and 2 more: https://git.openjdk.org/jfx/compare/cd76c4fa...f917d18e

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1394/files
  - new: https://git.openjdk.org/jfx/pull/1394/files/6abc47b1..f917d18e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1394&range=09
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1394&range=08-09

  Stats: 25230 lines in 281 files changed: 23351 ins; 339 del; 1540 mod
  Patch: https://git.openjdk.org/jfx/pull/1394.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1394/head:pull/1394

PR: https://git.openjdk.org/jfx/pull/1394