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