Implement possibility to disable debug output

Description

Because it requires quite a lot of computational power. This adds enabling/disabling debug output during runtime. TODO:

• Is this independent of the build option (Debug vs. Release)?
• What is the best default (enabled or disabled)?
• Should all DEBUG() messages replaced or only performance critical?

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• ~~Fixes: ~~

When external dependencies are removed

• Reporting to ...