Add recurring event support

[Input-BDF]

Recurring events store their information as mutiple components but share same uid. Therfore the components have a 'RECURRENCE-ID' cmp 3.8.4.4. Recurrence ID

This property is used in conjunction with the "UID" and "SEQUENCE" properties to identify a specific instance of a recurring "VEVENT", "VTODO", or "VJOURNAL" calendar component.

As for now only ´CONFIRMED´ VEVENT are transfered there need to be some change in:

ILSC-Chronos/src/core/app_factory.py

Line 249 in 07b9a05

def populate_from_vcal_object(self):

ILSC-Chronos/src/core/app_factory.py

Line 252 in 07b9a05

self.uid = str(self.ical.get('uid'))

and

ILSC-Chronos/src/core/app_factory.py

Line 566 in 07b9a05

event.calDAV = calEvent

Think event.calDAV has to get assigned component to get this work which furthermore involves more changes in class ILSCEvent
But by this way making 'beautyfications' on the source event may break completely for this kind of VEVENTS.

EDIT: Recheck icalendar_instance and subcomponents on this. Tricky point is the recheck from target to source object(s)