Multiple concurrent composers

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud user I want to be able to write a message while I'm writing another one. For example, I'm writing a long report, then I remember to send my colleague a quick reminder, then resume the report. Right now I have to fully park the email as draft and look it up in my drafts.

Describe the solution you'd like

Allow me to have more than one minimized composer session.

Describe alternatives you've considered

N/a

Additional context

This is a follow-up to https://github.com/nextcloud/mail/issues/8338