StandardActionManager: allow using a proxy on top of FavoriteCollectionsModel

Summary:
Using favoriteSelectionModel->model(), we get the correct model
for the selection.

Test Plan:
Fixes asserts when selecting a favorite, with the upcoming
kmail patch to add a proxy on top of FavoriteCollectionsModel.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8803