Defines an attribute for an interface element.
void IupSetfAttribute(Ihandle *element, char *a, char *f, ...); [in C] [There is no equivalent in Lua]
element: identifier of the interface element.
a: name of the attribute.
f: format that describes the attribute. It follows the same standard as the printf function in C
...: values of the attribute.
This function is very useful because we usually have integer values and want to pass them to IUP attributes, but this is done by means of a string. This way, we can commonly use sprintf to compose that string.