New new SDDM theme

Summary:
Implements the new SDDM theme

Code is written with the intention to easily port the lock screen
All SDDM related code is now shifted into Main with Login.qml acting as
an abstraction layer.

Test Plan: Ran it, against the awesome mock data.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: bshah, mart, apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2399