Update plugins.qmltypes for QtQuick.Layouts

Change-Id: I9b717b9986ce27102ab4a1bd8f6d6cc9aae006f1
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Don't register LayoutBase as a type.

Use qmlRegisterRevision() instead of qmlRegisterUncreatableType()
This avoids that qmlplugindump generate an entry for LayoutBase and
consequently doesn't show up in the code completion in Qt Creator.

Change-Id: I8688b5d05382c3fbc3fc526cfbf34eda05b35d77
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>