Make calendar priting work with any KCalendarCore::Calendar

The printing code depends on ETMCalendar only because the ETMCalendar
precalculates map of related Todos when their are inserted into the
Calendar and exposes them through an API. The printing code makes
use of this feature to print related Todos.

With this change the printing code uses the generic
KCalendarCore::Calendar and to preserve the functionality, it simply
iterates through all incidences in the calendar and tries to find any
Todo related to the current Todo. While less efficient, it only happens
when printing a calendar (no-body does that) and one would have to have
a really huge calendar to notice any big delay. Additonally, we can
upstream the functionality of retrieving related Todos into
KCalendarCore::Calendar itself, since that feature does not depend on
Akonadi at all.