implement shortcut to toggle filter view
Summary: use the shortcut as in dolphin to toggle the filter view. also simplifies the implementation a bit and adjust tests
Test Plan: filter view is toggled persistently across all views either with shortcut or button. test passes
Reviewers: Elisa, mgallien
Reviewed By: Elisa, mgallien
Tags: Elisa
Differential Revision: https://phabricator.kde.org/D11673