Let ApplicationWindows default max size be unbounded.

Previously, the ApplicationWindow  determined its maximum size from
its content items maximum size. This was inconsistent with how QWidgets
normally behaved. The default max size of an ApplicationWindow is now
therefore unbounded.

If the application want the ApplicationWindow to have a maximum width or
height, the application must set up the binding manually. This can be
done with a fixed maximum size or it can be bound to a layout:

contentItem.maximumHeight: grid.Layout.maximumHeight
contentItem.maximumWidth: grid.Layout.maximumWidth

If the grid layout has anchors.margins, those need to be taken into
consideration too, resulting in the following binding:

contentItem.maximumHeight: grid.Layout.maximumHeight +
                           grid.anchors.topMargin +
                           grid.anchors.bottomMargin

Change-Id: Ibac80ba71f12e3eb9e5db8f875ecfa64b1e28b0d
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>