Make the state of looping (on/off) as well as the looping region session-based

### Problem to be solved

In DAWs, loop/cycle region and loop state are normally saved as part of the project/session. Audacity doesn't save the state of the looping.

This is especially important now that we have the loop region export feature, because the loop region defines the render/export target.

Currently, Audacity _does_ save the looping region as a selection but only when you close and re-open the project; if you quit and restart Audacity, the region will be lost.

### Your idea

- We need to remember the state of the looping (on/off) for each project.
- We need to remember the looping region each time Audacity is launched, as well as between sessions.