[evita] ui: Introduce |ui::EventHandler| and |ui::EventTarget|.
[evita] ui: Rename |ui::KeyboardEvent| to |ui::KeyEvent| to follow Chromium class name.
[evita] ui/events: Change event target type to |ui::EventTarget|.
[evita] ui/events: Introduce |ui::EventType::MouseEntered| with |ui::Widget::OnMouseEntered(const ui::MouseEvent&)|.
[evita] ui/controls: Utilize |ui::EventHandler::OnKeyEvent()| and |ui::EventHandler::OnMouseEvent()|.