Do not quit while the editor is open

[lucc]

Describe the bug
Alot allowes me to :quit even though an external editor is still running.

Software Versions

• Python version: 3.8
• Alot version: 0.9.1-38-ga814a774

To Reproduce
Steps to reproduce the behaviour:

1. set editor_in_thread=True (and for me also editor_spawn=True, might depend on
   your editor) in the config
2. compose an email
3. execute :quit while the editor is still running
4. you now have an open editor with a tmp file that is useless but might
   contain a long mail you just composed. Alot/notmuch do not know about this
   draft/work any longer and will not pick it up automatically then you
   restart alot

Propsed solution
Ideally alot would not quit. In my opinion this does not even have to have
its own option but can be implemented unconditionally. I never would want to
lose this tmp buffer and would be happy with the "overhead" of being forced to
close the editor first and close the envelope buffer.

This would align with the UX of having the options set to false where the
editor hides the UI of alot so I can not quit accidentally.