Simplify CSS API Builder hierarchy

[Schahen]

Currently we have:

• interface CSSBuilder : CSSStyleRuleBuilder, GenericStyleSheetBuilder<CSSBuilder>
• class CSSBuilderImpl : CSSRuleBuilderImpl(), CSSBuilder, CSSRulesHolder by rulesHolder
• interface CSSStyleRuleBuilder : StyleBuilder
• open class CSSRuleBuilderImpl : CSSStyleRuleBuilder, StyleBuilderImpl()
• interface StyleBuilder
• interface CSSRulesHolder
• interface GenericStyleSheetBuilder<TBuilder> : CSSRulesHolder

We need to revisit all this entities and do following:

1. Remove all entities that make sense to remove
2. Make sure only entities that are intended to be public are public
3. If the final set of entities is still big and confusing - document the hierarchy and explicitly justify it.

[okushnikov]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.