Don't exclude virtual calendar collections from ETMCalendar

KOrganizer adds some events to "Open Invitations" and "Declined
Invitations" virtual collections under "Search". Since KOrganizer
does not support single event being present in multiple Collections
(a limitation in KCalCore really), KOrganizer in some cases does
not show unaccepted invitations and there is no way to get to them.

This change allows virtual collections to be shown in ETMCalendar,
so users can enable those virtual "calendars" in KOrganizer to at
least see the invitations, until the underlying issue of multi-instance
events is fixed.

CCBUG: 361419