pyside-uic should save a reference to QSpacerItem

When a ui file from designer is converted into python with pyside-uic,
convenient references are saved in the generated class for referencing widgets
and layouts. Widgets and layouts are also named with setObjectName(). This is
helpful since python code can later access and programmatically alter these
objects. QSpacerItem is treated differently, though. Instances are created as
local variables in the setupUi() method, and no objectName is set. It would be
nice if spacers were handled like widgets and layouts.