A style selector that matches any element with the
specified child ID and parent ID. For example, if
an element ID is "slider.thumb.button" then there
are several "parent"/"child" relationships:
- slider, thumb
- slider, button
- thumb, button
A ContainsSelector of ("slider", "button") would match
any of a slider's buttons, to include the thumb or
any arrow buttons, etc..
Note: mostly callers do not have to worry about
the specifics of this class as there are methods
on Styles that create these selectors internally.