IncidenceEditor: pass ETM to CollectionCombobox, if it was set

Summary:
If it's nullptr, then CollectionCombobox will create an ETM internally,
juts like before.

Depends on D28556

Test Plan: incidence dialog is much faster

Reviewers: vkrause, mlaurent, winterz

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28557