Rework MenuBar. Add MenuButton. Add mouse event cascading to floaters.
LinearLayout: spacing now applies to all children, not just those with expand.
Fix Window resize request inside layouts.
UnicodeGraphics: prepare for styling/theming.
Rework layouts: Layout is now normal Container which places its children upon resize event.
Drop TopWindow, top is now any subclass of Container.
Add floater concept: floaters are widgets drawn over normal widgets, not clipped by parent.
Add HScrollbar and Scrollbar abstract base class.