[CSS] Remove unused stacking contexts from ViewCSS

(StackingContext::~StackingContext) : Detach this context from the tree of the stacking contexts.
(StackingContext::detach): New
(CSSStyleDeclarationImp::revert): If this style has its own stacking context, detach it from the stacking contexts.
(CSSStyleDeclarationImp::clearStackingContext) : New
(ViewCSSImp::constructComputedStyle, ViewCSSImp::calculateComputedStyle) : Regenerate a stacking context if there isn't one