Change some interface into member variables

[PetrFlajsingr]

class ButtonBase : public ItemElement, public Clickable {
// ...
};

to

class Button : public ItemElement {
  using ClickEvent = ClassEvent<Button>;
public:
  ClickEvent clickEvent;
};

---

class ItemElement : public ElementWithID, public Focusable, public Hoverable {
// ...
};

to

class ItemElement : public ElementWithID {
  using FocusChangeEvent = ClassEvent<ItemElement, bool>;
  using HoverChangeEvent = ClassEvent<ItemElement, bool>;
public:
  FocusChangeEvent focusChangeEvent;
  HoverChangeEvent hoverChangeEvent;
};

etc.