Mail · Boxes and lists: mailboxes, folders and labels, the mail list, inbox zero

MailboxList sets mailboxes, folders and labels in sections: a press, an
arrow or Enter opens one, the open one lit; a box with mail unread
stands out with its count, and a label wears one of the theme's chart
hues. MailList holds messages in a multiple-select list: the sender
strong beside a dot while unread, the subject and first words under
it, the time today, the weekday this week, the date this year or with
its year, a paperclip for files, and a star whose press stays off the
row. InboxZeroState points to feedback::EmptyState.

Sections of selectable lists now have one home, lists::Sections, which
ChannelList and MemberList use too; ListItem gains a third line.

src/primitives/icon.rs

@@ -218,6 +218,7 @@ icons! {
     Music => "music",
     Network => "network",
     NotebookPen => "notebook-pen",
+    OctagonAlert => "octagon-alert",
     Package => "package",
     Palette => "palette",
     PanelBottom => "panel-bottom",