RFR: JDK-8090647: Mnemonics : on windows we should cancel the underscore latch when an app loses focus.

[John Hendrikx]

This fix hides mnemonics when the Scene's Window loses focus.

Before integration, we need to make sure this is correct behavior for Mac and Linux as well. It is correct for Windows.

How to test:

- Create a control with a mnemonic
- Alt-tab to another window (mnemonics appear when alt is pressed)

Before:
- Window shows visible mnemonic still and reacts on them when returning

After:
- Window hides mnemonics when focus is lost and doesn't react on them when returning

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Commit messages:
 - Mnemonics should be hidden when Scene's window loses focus

Changes: https://git.openjdk.org/jfx/pull/1030/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1030&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8090647
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1030.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1030/head:pull/1030

PR: https://git.openjdk.org/jfx/pull/1030