use std::function<void()> instead of global callbacks

[baggior]

Description

[Describe what this change achieves]

Issues Resolved

[List any existing issues this PR resolves; include Fixes #xxx or Closes #xxx (where xxx is issue number)]

Check List

General

• Code follows coding style defined in STYLE.md
• Doxygen comments are included inline with code
• No unnecessary whitespace; check with git diff --check before committing.

The following have been modified to reflect new features, if warranted

• README.md
• keywords.txt (use tabs as whitespace separators)
• library.properties
• examples/ - update or create new ones, as warranted

The following have NOT been modified

• doc/ - will be updated upon versioned release
• .ruby-gemset
• .ruby-version
• CHANGELOG.md - will be updated upon versioned release (HISTORY.md is deprecated)
• Gemfile
• LICENSE
• VERSION - will be updated upon versioned release

NOTE: use ?w=1 to see the changes and ignore whitespaces
eed84d3?w=1

[baggior]

can you merge this pool reqst?