calendar: Only show section title for other days

The section titles usually don't provide a lot of value - messages
themselves are usually pretty unambiguous about their type, and
having a hidden shortcut to some settings panel or application isn't
essential either - except when showing the selected date when browsing
other days, as it adds context to the listed events. Based on that,
remove the section title as a general MessageListSection feature and
move it into the EventsSection, where we only show it when it is useful.

https://bugzilla.gnome.org/show_bug.cgi?id=775763