Restrict fields in mu4e-view-fields. Make the defcustom a bit more
helpful by checking for valid fields.

Add :labels to default fields

Remove some unused code.

Support showing the icons in the message view as well.
(see previous commit)

In mu4e-view-mime-part-action, support icons or, well, "icons".

For this to work user needs to install a package like nerd-icons and set
mu4e-file-name-to-icon-function to #'nerd-icons-icon-for-file. (see
docstring for details)

Remember the exact frame we created, and only kill that one.

Ensure mu4e-compose-post-hook fires at most once.

mu4e-compose-reply-include-mime-types specifies the mime-types for
message attachments that should be included in replies, with the default
set to "text/x-patch" -- i.e., patches are included in replies, so you
can comment on them.

As mentioned in #2896.