Use EntityCollectionOrderProxyModel for filtering folders.

Summary:
This way the FolderTreeWidgetProxyModel (its source) doesn't get
filtered, which will allow FavoriteCollectionsModel to not get filtered
by the filter lineedit (once it uses FolderTreeWidgetProxyModel
directly).

This also simplifies FolderTreeWidgetProxyModel.

Test Plan: lineedit filtering in kmail still works

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8818