[File Tree] Add "Open Containing Folder" menu entry
Summary: It was present in the Projects view but not the Documents tree.
Test Plan:
Reviewers: Kate, VDG, dhaumann
Reviewed By: Kate, VDG, dhaumann
Subscribers: dhaumann, abetts, ngraham, kwrite-devel
Tags: Kate
Differential Revision: https://phabricator.kde.org/D19197