Gets 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
read it fails.
For convenience, some of the most common attributes have been
exposed directly as properties, for example PPGItem.Label and PPGItem.UIItems.