Implement reordering of favorite folders using DnD

Summary:
In order to separate favorites from the "entity order" QSFPM
used by the folder tree widget (it was always weird that filtering or
changing the sort order for the folder tree widget would affect favorites),
we need the favorites to get their own "ordering" proxy (otherwise
ordering is random, based on ETM population order). So while at it,
I implemented manual reordering of favorites.

BUG: 376073
FIXED-IN: 18.04

Test Plan:
I can reorder favorites (in listview mode) using DnD
and the order is persisted

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8884