RFR: 8296387: [Tooltip, CSS] -fx-show-delay is only applied to the first tooltip that is shown before it is displayed [v5]

[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 tests for the behaviour since there were none before.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  Add more documentation and improve css stylesheet test threshold

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1394/files
  - new: https://git.openjdk.org/jfx/pull/1394/files/5af893c1..a3f9f609

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1394&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1394&range=03-04

  Stats: 18 lines in 4 files changed: 8 ins; 4 del; 6 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