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

[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.

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Commit messages:
 - JDK-8307208: Add GridPane constructor that accepts hGap and vGap values

Changes: https://git.openjdk.org/jfx/pull/1125/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1125&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307208
  Stats: 25 lines in 2 files changed: 25 ins; 0 del; 0 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