Add ability to extend base only sections

[nvdaes]

Link to issue number:

Fixes #7467

Summary of the issue:

Though CONFIG.CONF.BASE_ONLY_SECTIONS is a public constant intended to be extended of add-ons, this is not really posible without using hacks like resetting the configuration, what can produce side effects.

Description of user facing changes:

None.

Description of developer facing changes:

Add-on developers can add sections which Will be applied to the base configuration, not other profiles.

Description of development approach:

• Added a yaml file to write base only sections included in add-ons. Developers should use the onInstall function of installTasks.
• _initBaseConf from ConfigManager Will load sections contained in the yaml file, and these sections Will be added to `BASE_ONLY_SECTIONS.
• Created an addSection function in the ConfigManager class.

Testing strategy:

None yet. I plan to add unit tests and test this manually with add-ons.

Known issues with pull request:

None.

Code Review Checklist:

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