### Link to issue number:

Implements one aspect from feature requests in nvaccess#6915

 ### Summary of the issue:

Commit 46a3436
("soffice: Report keyboard-triggered formatting toggles in Writer (nvaccess#16413)")
implemented announcement of keyboard-triggered formatting changes
in LibreOffice Writer, including the one to toggle underline.
(When English keyboard layout and UI, the shortcut is Ctrl+B.)

Issue nvaccess#6915 (among others) requests announcement also for
"toggle double underline", which is done via the Ctrl+D
keyboard shortcut for an English UI.

 ### Description of user facing changes

When toggling double underline in LibreOffice Writer using the corresponding keyboard shortcut, NVDA announces the new state ("double underline on"/"double underline off"). (nvaccess#6915, @michaelweghorn)

 ### Description of development approach

Add Ctrl+D to the list gestures handled by the existing gesture handler.

 ### Testing strategy:

1. start NVDA
2. start LibreOffice Writer (with English UI and keyboard layout)
3. Press Ctrl+D to enable double underline
4. Verify that NVDA announces "double underline on"
5. Press Ctrl+D to disable double underline again
6. Verify that NVDA announces "double underline off"
7. Press Ctrl+U to enable (normal/single) underline
8. Verify that NVDA announces "underline on"
9. Press Ctrl+D to enable double underline
10. Verify that NVDA announces "double underline on"

 ### Known issues with pull request:

None.

 ### Code Review Checklist:

- [x] Documentation:
  - Change log entry
  - User Documentation
  - Developer / Technical Documentation
  - Context sensitive help for GUI changes
- [x] Testing:
  - Unit tests
  - System (end to end) tests
  - Manual testing
- [x] UX of all users considered:
  - Speech
  - Braille
  - Low Vision
  - Different web browsers
  - Localization in other languages / culture than English
- [x] API is compatible with existing add-ons.
- [x] Security precautions taken.

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