RFR: JDK-8090647: Mnemonics : on windows we should cancel the underscore latch when an app loses focus. [v2]

[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

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Add test to verify scene disables mnemonics on focus lost

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1030/files
  - new: https://git.openjdk.org/jfx/pull/1030/files/8d4c33d6..31536a3c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1030&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1030&range=00-01

  Stats: 27 lines in 1 file changed: 27 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