Ergonomics

### Enhancement Summary

In TripleOscillator, it would be more convenient to experiment with different waveforms if the selection of waves was carried out not through the file opening dialog box, but using a list that leaves when you click "+" to the right of the window and remains there. Clicking on the list item should have the same effect as when opening the file, and the focus should go to the window in such a way that you can hear the sound by pressing the keys on the keyboard. The list should contain: 1) the selection of the folder whose files will be the items of the list and 2) the search field by name (possibly by other criteria).
Thus, it will take less clicks to try different waveforms.

#### Justification

#### Mockup

Start by locating TripleOscillator and tracing how waveform files are currently selected through the file-opening dialog. Done means waveform selection uses a persistent list with folder selection and name search, clicking an item has the existing loading effect, and keyboard focus returns to the instrument so its sound can be played.

Written by the indexing model from the issue text.