A style selector that matches a specific
element ID. Note: the selector may still refer
to parts of a composite element ID and "children" in
that case will inherit attributes of their parents.
In other words, using an ElementSelector("thumb") to
set attributes for a particular style can affect all
thumbs in any composite component, slider.thumb, scroll.thumb,
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.