expose the wallpaper config to the lockscreen

Summary: the lockscreen can change behavior based on the wallpaer (D11928)

Test Plan: wallpaper pluginmame is accessible to the greeter qml

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12314