[File Tree] Add "Open Containing Folder" menu entry

Summary: It was present in the Projects view but not the Documents tree.

Test Plan: {F6627981}

Reviewers: #kate, #vdg, dhaumann

Reviewed By: #kate, #vdg, dhaumann

Subscribers: dhaumann, abetts, ngraham, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D19197