Fix RMB/"Update Folder" being disabled on favorite collection

Summary: (when it's being filtered out from the foldertreeview)

Test Plan: type "zzz" in filter lineedit for folder tree, then try "Update Folder" in the context menu for a favorite, it was disabled.

Reviewers: vkrause, dvratil, mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14141