RFR: 8341372: BackgroundPosition, BorderImage, BorderStroke, CornerRadii should be final

[Michael Strauß]

Backgrounds and borders are comprised of immutable and final types. The following types are marked with the `final` modifier:

* Background
* BackgroundFill
* BackgroundImage
* BackgroundSize
* Border
* BorderWidths

The following types are not marked with the `final` modifier:

* BackgroundPosition
* BorderImage
* BorderStroke
* CornerRadii

This is probably just an oversight, as there is no value in allowing a subsection of these types to be extended.

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Commit messages:
 - Make BackgroundPosition, BorderImage, BorderStroke, CornerRadii final

Changes: https://git.openjdk.org/jfx/pull/1587/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341372
  Stats: 12 lines in 4 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jfx/pull/1587.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1587/head:pull/1587

PR: https://git.openjdk.org/jfx/pull/1587