Release candidate 2.2.0-rc1

[rockowitz]

Branch 2.2.0-rc1 contains release candidate 1 for ddcutil 2.2.0.

Given the extent of changes, the upcoming release is numbered 2.2.0, not 2.1.5. The current development branch is 2.2.0-dev, not 2.1.5-dev.

Release 2.2.0 contains major enhancements, particularly for display change detection, numerous small changes, and a large number of bug fixes. For details, see file CHANGELOG.md in the source code and web page Notes for Upcoming ddcutil Release 2.2.0 (draft).