At the moment, only the inbox of each mail account is added to neomutt.
This inbox is always called "Inbox", so if you configure multiple
accounts, it is hard to know which one is which.
This change allows the user to specify a display name per account that
uses `named-mailboxes` under the hood.
Additionally this change now allows to add other folders than the inbox,
for example the Trash, Spam or Drafts folders to be added on a per-account
basis. Using extraOptions is not possible here, as those are lazily
loaded on mailbox open and thus would appear at the bottom and not sorted
by account.
This commit also changes the default sidebar format string to use %D
instead of %B because %B will ignore named mailboxes and show the folder
name instead.
