Create a window per screen

A big window for all screens may hit hardware limits and it often has
a bad alignment as reported by #265.

Creating a window per screen doesn't suffer from these problems and,
as a nice side effects, it helps the Wayland port.

Porting notes for theme authors:

The "screenModel" context property was removed, we now have a new
"primaryScreen" boolean context property.

Your QML file is now loaded for each screen.

Create the background item without a repeater and anchors it to fill
the parent.

The greeter must be a component loaded by a Loader only when
"primaryScreen" is true.

See the themes shipped by SDDM for reference.

[ChangeLog][Greeter][Behavioral Change] Create a window per screen.