Add support for .SetIcon() for more components

[daveaglick]

Ideally just need to implement IHasIconExtensions but need to be careful that adding the icon as a child works in each case.

[daveaglick]

Another reminder to be careful about adding the icon as a child component - see #28. If the component with IHasIconExtensions generates a wrapping component that should actually contain the icon then the icon will be added to the nested component and not the wrapping component. It should be the responsibility of the IHasIconExtensions implementer to make sure this works out - see the fix for ButtonDropdown.