Enable and disable UI controls properly based on sharing status

Summary:
When a folder isn't shared, all of the UI controls that depend on it being shared
should be disabled.

BUG: 407500
FIXED-IN: 19.04.1

Test Plan: {F6828673, size=full, loop=true, autoplay=true}

Reviewers: #vdg, filipf

Reviewed By: #vdg, filipf

Differential Revision: https://phabricator.kde.org/D21253