Don't trigger SwipeListItem's actions unless they are exposed

Summary: I seem to be able to click through a SwipeListItem's delegate and trigger actions that should be out of reach. There are probably better ways to do this but this fixes it for me.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: ngraham, mart, apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10392