Focus
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Only one widget can have focus at the time.
Top widget has link to currently focused widget in 'focuswidget'.

Events emitted on change: focus, unfocus

mousedown - focus widget under mouse

tab - focus next child in container (depends on canfocus())
shift-tab - previous child

hide() -> unfocus

tab/shift-tab into / out off containers?
trapfocus # if True, tab cycles inside container

widget.hasfocus()