Fwupd-Backend Integration

Summary:
This patch brings the initial integration of Fwupd Backend.
    * This backend was implemented as the part of my GSoC"18 Project, its link can be found here:

    https://summerofcode.withgoogle.com/projects/?sp-search=abhijeet#5140246090481664

    * The status report for this backend can be found here:

    https://community.kde.org/GSoC/2018/StatusReports/AbhijeetSharma

Reviewers: apol, davidedmundson

Reviewed By: apol

Subscribers: zzag, anthonyfieroni, plasma-devel

Tags: #plasma, #discover_software_store

Differential Revision: https://phabricator.kde.org/D14050