RFR: 8311895: CSS Transitions [v21]

[Michael Strauß]

> Implementation of [CSS Transitions](https://gist.github.com/mstr2/c72f8c9faa87de14926978f517a6018a).
> 
> ### Future enhancements
> CSS transitions requires all participating objects to implement the `Interpolatable` interface. For example, targeting `-fx-background-color` only works if all background-related objects are interpolatable: `Color`, `BackgroundFill`, and `Background`.
> 
> In a follow-up PR, the following types will implement the `Interpolatable` interface:
> `LinearGradient`, `RadialGradient`, `Stop`, `Background`, `BackgroundFill`, `BackgroundImage`, `BackgroundPosition`, `BackgroundSize`, `BackgroundStroke`, `BorderWidths`, `CornerRadii`, `Insets`.
> 
> ### Limitations
> This implementation supports both shorthand and longhand notations for the `transition` property. However, due to limitations of JavaFX CSS, mixing both notations doesn't work:
> 
> .button {
>     transition: -fx-background-color 1s;
>     transition-easing-function: linear;
> }
> 
> This issue should be addressed in a follow-up enhancement.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  document css parser limitation

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/870/files
  - new: https://git.openjdk.org/jfx/pull/870/files/f64ad706..8a938dc8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=870&range=20
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=870&range=19-20

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/870.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/870/head:pull/870

PR: https://git.openjdk.org/jfx/pull/870