Signals for GUI

[michaelheilmann]

The GUI shall use the signals library.
For instance, a button typically provides the clicked signal:

class Button ... {
    ....
public:
   idlib::signal<void()> Clicked;
   ....
};

Reasonable class hierarchies will drastically reduce the number of signals a component at the bottom of the component hierarchy has to define: For instance, a button typically derives from a component that already defines the signals Signal<void()> TextChanged; and Signal<void()> FontChanged;.