Fix selecting a favorite that is filtered out in the foldertreeview.

Summary:
Adapt to the messagelib commit that requires a modelindex in
Pane::setCurrentFolder.

Also avoid calling setCurrentFolder twice when selecting a favorite
(once because the favorite selection changed, and once again because the
foldertreeview selection changed). This issue was pre-existing but
wasn't visible because it didn't populate the messagelist or created new
tabs.

Test Plan: Same as https://phabricator.kde.org/D14139.

Reviewers: vkrause, dvratil, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D14140