Fix for: BHV-8483 & BHV-8471

Targeting multiple issues found, may require additional fixes for other related problems to completely resolve the noted issue.

Fixes problem that allows invalid CSS styles to be applied via `applyStyle()`.
Fixes problem when using `componentOverrides` that stomped on base-kind's inherited observers.
Fixes problem where widgets were expecting `contentChanged()` to be called directly during initialization.