Make it possible to show the QR of a network connection
It's useful to be able to share a network connection by showing its QR
on the device so others can take a picture of it.
It shows a maximised window that will disappear when we click on it.
Test Plan: Looked at my wifi's QR
Reviewers: Plasma, jgrulich, ngraham
Reviewed By: jgrulich, ngraham
Subscribers: broulik, ngraham, plasma-devel
Differential Revision: https://phabricator.kde.org/D22457