FileItem plugin for opening and closing Vaults from Dolphin

Summary:
Adds a context menu item when the user right-clicks a directory that is a registered vaults mount point.

BUG:388602

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19991