RFR: 8273743: KeyCharacterCombination for "+" does not work on US QWERTY keyboard layout

[Martin Fox]

There was a mismatch between the way `get_glass_key` generated the Java key code for a given key press and how `getKeyCodeForChar` determined the key code for the matching character. For example, when pressing the plus sign on a US keyboard `get_glass_key` correctly generated `KeyCode.EQUALS` but `getKeyCodeForChar("+")` generated `KeyCode.PLUS`.

In this PR `getKeyCodeForChar` mirrors the behavior of `get_glass_key`; it determines which key the character lies on and generates a key code based on the unshifted character on  the same key.

I'm working on a more comprehensive test case that allows you to press any key on the keyboard and test whether a KeyCharacterCombination for that character will succeed or not. I've attached it to this thread. It might be worth submitting as a manual test case
[CharComboTest.txt](https://github.com/openjdk/jfx/files/7247265/CharComboTest.txt)
.

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Commit messages:
 - Query the current keyboard layout and key to correctly respond to getKeyCodeForChar.

Changes: https://git.openjdk.java.net/jfx/pull/635/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=635&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273743
  Stats: 82 lines in 1 file changed: 63 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jfx/pull/635.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/635/head:pull/635

PR: https://git.openjdk.java.net/jfx/pull/635