RFR: 8307208: Add GridPane constructor that accepts hGap and vGap values [v2]

[Marius Hanl]

> This PR adds a new constructor to `GridPane` that accepts a `hgap` and a `vgap`.
> As also written in the ticket, this is a small enhancement to improve the creation of a layout involving a `GridPane` without FXML.
> In my experience, the `hgap` and the `vgap` are almost always set when creating a layout with the `GridPane`, as a space between the content just looks much better.
> 
> This also fits well with other classes like `HBox` and `VBox`, which also provide a constructor that accepts the spacing.

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

  JDK-8307208: Improve javadoc and change copyright year

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1125/files
  - new: https://git.openjdk.org/jfx/pull/1125/files/76d5be3f..570061a4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1125&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1125&range=00-01

  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1125.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1125/head:pull/1125

PR: https://git.openjdk.org/jfx/pull/1125