Add configurable mailbox sort order

Closes #25

```
     sort_order unsigned integer           (optional) Override sort order on the sidebar for this mailbox.  Example:

                                           [accounts."imap.example.com".mailboxes]
                                             "INBOX" = { index_style = "plain" }
                                             "INBOX/Sent" = { sort_order = 0 }
                                             "INBOX/Drafts" = { sort_order = 1 }
                                             "INBOX/Lists" = { sort_order = 2 }
```

docs/meli.conf.5

@@ -457,6 +457,17 @@ Example:
   "INBOX" = { index_style = "plain" }
   "INBOX/Lists/devlist" = { autoload = false, pager = { filter = "pygmentize -l diff -f 256"} }
 .Ed
+.It Ic sort_order Ar unsigned integer
+.Pq Em optional
+Override sort order on the sidebar for this mailbox.
+Example:
+.Bd -literal
+[accounts."imap.example.com".mailboxes]
+  "INBOX" = { index_style = "plain" }
+  "INBOX/Sent" = { sort_order = 0 }
+  "INBOX/Drafts" = { sort_order = 1 }
+  "INBOX/Lists" = { sort_order = 2 }
+.Ed
 .El
 .Sh COMPOSING
 Composing specific options

melib/src/conf.rs

@@ -87,6 +87,8 @@ pub struct MailboxConf {
     pub ignore: ToggleFlag,
     #[serde(default = "none")]
     pub usage: Option<SpecialUsageMailbox>,
+    #[serde(default = "none")]
+    pub sort_order: Option<usize>,
     #[serde(flatten)]
     pub extra: HashMap<String, String>,
 }
@@ -99,6 +101,7 @@ impl Default for MailboxConf {
             subscribe: ToggleFlag::Unset,
             ignore: ToggleFlag::Unset,
             usage: None,
+            sort_order: None,
             extra: HashMap::default(),
         }
     }

src/conf/accounts.rs

@@ -2276,24 +2276,41 @@ fn build_mailboxes_order(
         }
     }
 
+    macro_rules! mailbox_eq_key {
+        ($mailbox:expr) => {{
+            if let Some(sort_order) = $mailbox.conf.mailbox_conf.sort_order {
+                (0, sort_order, $mailbox.ref_mailbox.path())
+            } else {
+                (1, 0, $mailbox.ref_mailbox.path())
+            }
+        }};
+    }
     tree.sort_unstable_by(|a, b| {
         if mailbox_entries[&b.hash]
-            .ref_mailbox
-            .path()
-            .eq_ignore_ascii_case("INBOX")
+            .conf
+            .mailbox_conf
+            .sort_order
+            .is_none()
+            && mailbox_entries[&b.hash]
+                .ref_mailbox
+                .path()
+                .eq_ignore_ascii_case("INBOX")
         {
             std::cmp::Ordering::Greater
         } else if mailbox_entries[&a.hash]
-            .ref_mailbox
-            .path()
-            .eq_ignore_ascii_case("INBOX")
+            .conf
+            .mailbox_conf
+            .sort_order
+            .is_none()
+            && mailbox_entries[&a.hash]
+                .ref_mailbox
+                .path()
+                .eq_ignore_ascii_case("INBOX")
         {
             std::cmp::Ordering::Less
         } else {
-            mailbox_entries[&a.hash]
-                .ref_mailbox
-                .path()
-                .cmp(&mailbox_entries[&b.hash].ref_mailbox.path())
+            mailbox_eq_key!(mailbox_entries[&a.hash])
+                .cmp(&mailbox_eq_key!(mailbox_entries[&b.hash]))
         }
     });
 
@@ -2302,22 +2319,30 @@ fn build_mailboxes_order(
         mailboxes_order.push(n.hash);
         n.children.sort_unstable_by(|a, b| {
             if mailbox_entries[&b.hash]
-                .ref_mailbox
-                .path()
-                .eq_ignore_ascii_case("INBOX")
+                .conf
+                .mailbox_conf
+                .sort_order
+                .is_none()
+                && mailbox_entries[&b.hash]
+                    .ref_mailbox
+                    .path()
+                    .eq_ignore_ascii_case("INBOX")
             {
                 std::cmp::Ordering::Greater
             } else if mailbox_entries[&a.hash]
-                .ref_mailbox
-                .path()
-                .eq_ignore_ascii_case("INBOX")
+                .conf
+                .mailbox_conf
+                .sort_order
+                .is_none()
+                && mailbox_entries[&a.hash]
+                    .ref_mailbox
+                    .path()
+                    .eq_ignore_ascii_case("INBOX")
             {
                 std::cmp::Ordering::Less
             } else {
-                mailbox_entries[&a.hash]
-                    .ref_mailbox
-                    .path()
-                    .cmp(&mailbox_entries[&b.hash].ref_mailbox.path())
+                mailbox_eq_key!(mailbox_entries[&a.hash])
+                    .cmp(&mailbox_eq_key!(mailbox_entries[&b.hash]))
             }
         });
         stack.extend(n.children.iter().rev().map(Some));