WIP 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.
I think I said earlier this won't work because Qt will automagically move the screen of the view to screen0 before this signal is emitted.

What you could do given we are now Qt5 is to use a lambda on it.
     connect(screen, SIGNAL(destroyed(QObject*)),
                this, [](view)(screenRemovedForView(view)));

that way you're passing the view, and it doesn't matter if the view's screen changes when you remove it.