Sets an attribute on the control. Each control
has a collection of attributes which can be used to tweak its
appearance or to provide it with additional states. Some of
these attributes are general and apply to almost any control,
for example siUIAlignCenter. Others are specific to the
control, for example siUINoSlider (which only applies to
numeric controls).
If the attribute argument is not a recognized attribute the call
to write it fails.
For convenience, some of the most common attributes have been exposed
directly as properties, for example PPGItem.Label
and PPGItem.UIItems.
