elm calendar: add mark_del method and move Elm_Calendar_Mark to .eo file

elm calendar: add mark_del method and move Elm_Calendar_Mark to .eo file

Summary:
Add mark_del as a method in the Elm.Calendar class.
Move the elm_calendar_mark_del() function (which do not receive an Eo object)
to elm_calendar_legacy.h.

Move the declaration of Elm_Calendar_Mark to elm_calendar.eo as an opaque
struct.
Rename it to Elm.Calendar.Mark to follow eolian standards.

Update documentation to refer to mark_del method instead of the
elm_calendar_mark_del() function.

Reviewers: felipealmeida, vinipsmaker, larryolj, cedric, tasn

Reviewed By: tasn

Differential Revision: https://phab.enlightenment.org/D3582