calendar: Split out message list base classes

Currently both the base classes for messages/sections and the message
list itself that instantiates the available sections are located in
the same module. As a result, it isn't possible to define sections
in a different module without introducing circular dependencies. The
Calendar module is already unwieldily large, so split it up a bit to
avoid it growing even bigger in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=756491

js/ui/dateMenu.js

@@ -360,7 +360,7 @@ const DateMenuButton = new Lang.Class({
         }));
 
         // Fill up the first column
-        this._messageList = new Calendar.MessageList();
+        this._messageList = new Calendar.CalendarMessageList();
         hbox.add(this._messageList.actor, { expand: true, y_fill: false, y_align: St.Align.START });
 
         // Fill up the second column