CLONE -SkinnableComponent shouldn't define "selected"

SkinnableComponent declares a 'selected' property even though, as far as I can tell, it's not used for anything. Subclasses that actually have a useful selected property, like ToggleButton, redeclare it themselves.

This causes problems for reflection-driven tooling. For example, we show a 'selected' property for Button even though you really need to use a ToggleButton for such functionality.