Implements the new SDDM theme.
There's a lot of hardcoded values, but the mockup asks for it.
It also doesn't match the mockup exactly because all grid unit sizes are just wrong and partly because in many cases I opted for just using the
Plasma default.
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.