RFR: 8087557: [Win] [Accessibility, Dialogs] Alert Dialog content is not fully read by Screen Reader [v2]

[Ambarish Rapte]

> Accessibility is not implemented for JavaFX dialogs. This change is to implement the accessibility on windows platform.
> Without this fix : On Windows platform, content of Dialog are not read by Windows Narrator or JAWS.
> With this fix:
> 1. Windows narrator reads the dialog correctly
> 2. JAWS fails to read the header text. : This will be handled separately 
> 3. Behavior of VoiceOver on MacOS remains unchanged : This will be handled separately.
> 
> Verification:
> 1. Run HelloAlert toy app, with fix.
> 2. Start Windows Narrator
> 3. Click on the different buttons to show different Dialogs
> => Observe that Narrator reads all the content correctly.

Ambarish Rapte has updated the pull request incrementally with one additional commit since the last revision:

  review comment : mac NPE

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Changes:
  - all: https://git.openjdk.org/jfx/pull/873/files
  - new: https://git.openjdk.org/jfx/pull/873/files/408561b4..39d18967

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=873&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=873&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/873.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/873/head:pull/873

PR: https://git.openjdk.org/jfx/pull/873