Audacity multi-track editor: accessibility enhancement

This add-on adds extra functionality when working with Audacity multi-track editor:

This addon's version has been tested with audacity 3.6.4 x64, 3.6.2 x64, 3.5.1 x64, 3.4.2 x64, 3.3.3 and 3.3.0. Prior versions of Audacity are not supported by this version.