Previously we kept a Selector object for every "simple selector" (term
from CSS spec). Now we keep one for every match operation. So given the
selector
".a b:focus"
we will have 4 elements:
- pseudoclass ":focus"
- element "b"
- match any desendant (the space)
- class ".a"
Each of those is represented by a "selector class" which is basically
the collection of vfuncs for this selector.
'window-unfocused' is too long and mentions "focus" which is historically
loaded with the meaning "input focus".
'backdrop' isn't generally used in GUI speak and still conveys the state the
widgets in an unfocused or background toplevel window are in.
Passing the length of the widget path that is relevant is not necessary
anymore, it was only useful for inheritance. Instead, we now pass the
state flags and let the selector do the state matching for us.
