Make it possible to show the QR of a network connection

Summary:
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

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22457