ENYO-1996: an `enyo.singleton()` cannot be an `enyo.Component`

because `enyo.XXX.setContainer` is only defined with `XXX` being
`UIComponent` and above (eg. `Control`)

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>