Shorten indentation beeps (#18898)
### Summary of the issue:
NVDA can be configured to report line/paragraph indentation with beeps.
However, the beeps are distractingly long, especialy at high speech
rates.
### Description of user facing changes:
The duration of line indentation beeps has been reduced. They are now
the same length as progress bar beeps.
### Description of developer facing changes:
The `IDT_TONE_DURATION` constant has been removed.
### Description of development approach:
Moved line indentation beep duration to config, so this customisation
may be surfaced if desired in future. Replaced the constant with a
getter method.
### Testing strategy:
Navigated code with line indentation set to beeps.
### Known issues with pull request:
None
