Add notification counters to bottom tab

A counter now increments in the bottom bar for unfocused windows showing how
many unread messages are pending. Tabs with no pending messages show [*]
instead of their index (showing the index is useless and somewhat confusing)

src/notify.c

@@ -126,6 +126,8 @@ static void tab_notify(ToxWindow *self, uint64_t flags)
     } else if ((flags & NT_WNDALERT_2) && (!self->alert || self->alert > WINDOW_ALERT_1)) {
         self->alert = WINDOW_ALERT_2;
     }
+
+    ++self->pending_messages;
 }
 
 static bool notifications_are_disabled(uint64_t flags)