Wayland supports multiple windows.

After this, QtQuick.Dialogs for example will use widget-based dialogs
instead of in-scene QML items.

Change-Id: I3ada4c59fc6ae40d4eb6d3eb3bde70e546d96e6d
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>