EmailAddressSelectionWidget: use ETM signal to expand, rather than 1s timer.

Summary:
This leads to much better use interface, the signal being emitted much
faster than 1s (it's emitted when the collections have been listed,
before the items themselves are listed), it also works around a crash
in QSFPM that happens sometimes when the listing isn't done after 1s
(sounds like ETM or one of the proxies is in an inconsistent state,
something which I still have to debug)

Test Plan: emailaddressselectionwidgettest no longer crashes for me.

Reviewers: mlaurent, dvratil

Reviewed By: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D7613