RFR: 8330304: MenuBar: Invisible Menu works incorrectly with keyboard arrows

[Andy Goryachev]

The root cause is that the skin used two fields to store one entity (`focusedMenu` and `focusedMenuIndex`), causing mismatch when invisible menu(s) are present.

The fix involves using a single index variable.

Also wanted to note that in theory, `openMenu` and `openMenuButton` can also be replaced with a single boolean, but I decided not to change these in order to keep the amount of diffs to a minimum.

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Commit messages:
 - 8330304 MenuBar: Invisible Menu works incorrectly with keyboard arrows

Changes: https://git.openjdk.org/jfx/pull/1467/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1467&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330304
  Stats: 80 lines in 3 files changed: 68 ins; 6 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1467.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1467/head:pull/1467

PR: https://git.openjdk.org/jfx/pull/1467