RFR: 8259680: Need API to query states of CAPS LOCK and NUM LOCK keys [v4]

[Kevin Rushforth]

> The JavaFX API does not provide a way to get the state of CAPS LOCK or NUM LOCK on the keyboard. Being able to read the lock state would allow an application to inform the user that caps lock was enabled for passwords or other usages where the keyboard input might not be echoed. It would also allow an application to do spell checking / auto-correction that might ordinarily be skipped when typing all upper-case letters.
> 
> We need an equivalent JavaFX API to the existing AWT `java.awt.ToolKit::getLockingKeyState` method. A natural place to put this in JavaFX is in the `javafx.application.Platform` class, so we propose to create a new `Platform::isKeyLocked` method, which will take a `KeyCode` -- either `CAPS` or `NUM_LOCK` -- and return an `Optional<Boolean>` indicating whether or not that key is in the locked or "on" state. If we can't read the key state on a particular platform, we will return `Optional.empty()`, rather than throwing a runtime exception as AWT does.
> 
> I have provided both an automated Robot test and a manual test. The latter is needed primarily because we can't set the CAPS lock on Mac using Robot, but also because we want  way to test the case where the user has enabled CAPS lock before the program starts.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  Updates based on review comments

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/385/files
  - new: https://git.openjdk.java.net/jfx/pull/385/files/c4961b16..0ae116f5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=385&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=385&range=02-03

  Stats: 8 lines in 3 files changed: 1 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/385.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/385/head:pull/385

PR: https://git.openjdk.java.net/jfx/pull/385