Add MemberSlot

[michaelcowan]

Add MemberSlot

Submitted as a potential work around for issue #8

MemberSlot extends Slot and is designed to update references to member functions as they move or are copied.

e.g.

class Popup {
public:
    MemberSlot<Popup> show = MemberSlot<Popup>(this, &Popup::render);
private:
    void render() {
        Popup * pointer = this;
    }
};

Popup popup;
Signal<> signal;
signal.connect(popup.show);

Popup copy = popup;

signal.emit();

Now, when emit is called, render() will be called on each instance of Popup

What has happened is that during the copy of MemberSlot, the callback function of show is recreated with a different value for this.

Calculating the value of this relies on the offset in bytes between the two MemberSlot instances being the same as the offset between the two Popup instances.