Add support to reduce/restore compose view to peek at messages list

[Kabouik]

Every now and then, someone arrives in my office and says "Did you see that email I forwarded?" More often than not, I'm currently composing a really long and tedious email and have no choice but postponing it to save it to drafts, go back to my email list, mark the email I was replying to as unread to not forget finishing later, then read that new email (what's the point of sending me emails and then asking me immediately in my office to read them when I didn't choose to, but anyway), then go back to my drafts, resume, continue. The whole thing can happen multiple times before I'm done with the original draft if it was really a tricky one.

This is 9 keybindings or keystrokes from the Postponing action to the Continue draft action. It would be really great if there was such as thing as "Freeze and reduce compose view to peek at message list". This would basically make it a two-keybindings action with "Freeze and reduce" and "Restore" action, which could even be on the same keybind.

I imagine however that nmail is probably not coded in a way that allows decoupling both views easily, but I'm asking anyway just in case.

[Kabouik]

Just adding a note to mention that this would also be helpful when the email being composed is a reply to multiple/many messages (a usecase I described recently on the Telegram group); although in that case a read-only terminal window being able to read those messages while the main window is used to compose would probably be more powerful (but also more demanding to set up and probably a bit more of a corner case).