fix(dropdown): Improvements on styles and handle menu visibility via CSS

document.hidden is used on dropdown focus and blur handlers to handle changes on screen visibility correctly
Instead of toggling the whole dropdown menu from DOM, we now use CSS to handle its visibility
Effect that listens selectOption to focus on it is removed and now we only use focusedOption to handle scroll computations
Direct DOM access to the focused list item element is removed to use React Ref API.
Nesting of dropdown-header-button CSS block is removed

studiobakers / react-ui-toolkit