Hide/Show automation tracks

**Enhancement Summary**
Option to hide/show automation tracks on song editor.

**Justification**
It would be a way to reduce the number of visible tracks on the song editor, hence simplifying the composition tasks. If a song has a lot instruments and samples, it gets complicated to "navigate" in the song editor, and it is not unusual to add two or three automation tracks on some instruments (sometimes I "reuse" an automation track for diferent parameters in order to reduce the number of tracks, but in the end the project gets messy). And normally, when an automation is set, we don't need to "see" it in the screen.
When hide is active, it's not possible to add or modify automation tracks, only when visible.

No files or tests are named. Start by locating the song editor and its automation-track handling, then trace how tracks are displayed and edited. Done means users can hide and show automation tracks, with hidden tracks unavailable for adding or modifying automation.

Written by the indexing model from the issue text.