simplify CssTextModel
StyleSheet.updatePropertyValue
maintain CssTextModel on StyleSheet and on RuleModel
getStyleSheetObjects()
getStyleSheetObjects async
RulePrechecker
calling rulePrechecker.testSave
push StyleSheets functionality down into StyleSheet
pass precheck save value to updatePropertyValue
logUpdateAttempt in case of error
RuleSaver.updatePropertyValue
fix new rule offset bug
fix this.ruleSavers.pushFollowingRulesAlong