Index: try description when changing mailboxes

Index: try description when changing mailboxes

Mailbox: find mailbox by description

Introduce `mutt_find_mailbox_desc()`, which takes a pointer to a
description, and tries to find a mailbox that corresponds to it.

With the introduction of `named-mailboxes` and removal of
`virtual-mailboxes` special cases, users may prefer to operate with
descriptions instead of (potentially) ugly paths. We can no longer
assume that a buffer is a path and require a method to find by
description.

This is not included as an `mxapi` function since `desc` is common to
all mailboxes so the backends do not need implement their own function.

Index: try description when changing mailboxes

Since mailbox descriptions are more prevalent with `named-mailboxes` and
`virtual-mailbox` special cases being removed, some users will try to
change folders with a description instead of a path.

This commit modifies `main_change_folder()` to look for a mailbox with a
given description if path probing fails.