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
it helps the Wayland port as a nice side effect.

Porting notes for theme authors:

Theme authors should keep in mind that the "screenModel" context
property is removed and a new "primaryScreen" boolean context
property is added.

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

The greeter now should be a component loaded by a Loader only when
"primaryScreen" is tree (unless you want a greeter for each and
every screen.

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