Enable recursive filtering in EntityOrderProxyModel

Summary: This allows to use it for the folder tree lineedit in kmail.

Test Plan:
No-op in itself (this proxy is currently only used for ordering),
but this commit allows mailcommon's foldertreewidget to use this proxy
for filtering, instead of filtering FolderTreeWidgetProxyModel
(which is shared with the favorites model, where we don't want filtering)

BIC of course, but I was told that's OK.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8799