Add environment-variable to be able to disable window-decorations

When QT_WAYLAND_DISABLE_WINDOWDECORATION is set, QWaylandWindow
doesn't create a QWaylandDecoration anymore for any type of window.
This is especially useful for (embedded) use-case where one doesn't
need window-decorations at all or want to have server-side decorations.

Change-Id: I413e4f3a96241e76b73364833e4ee56b6d9a1680
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Implement support for minimizing, as per recent core protocol additions

Change-Id: Iba29c0a9c8d15ebf7bea86798d2c5236fb808094