Tripple-click to select all audio

### Description

Tripple-click anywhere in project area should result in all audio selection (including empty spaces)

No file, test, or entry point is named in the issue. Start by locating the project-area mouse-selection entry point and existing selection tests; done means a triple-click anywhere in that area selects all audio, including audio separated by empty spaces, with the relevant tests passing.

Written by the indexing model from the issue text.