The define KWIN_SINGLETON adds to a class definition:

public:
    static Foo *create(QObject *parent = 0);
    static Foo *self() { return s_self; }
protected:
    explicit Foo(QObject *parent = 0);
private:
    static Foo *s_self;

There is an additional define KWIN_SINGLETON_VARIABLE to set a different
name than s_self.

The define KWIN_SINGLETON_FACTORY can be used to generate the create
method. It expands to:

Foo *Foo::s_self = 0;
Foo *Foo::create(QObject *parent)
{
    Q_ASSERT(!s_self);
    s_self = new Foo(parent);
    return s_self;
}

In addition there are defines to again set a different variable name and
to create an object of another inheriting class.

All the classes currently using this pattern are adjusted to use these
new defines. In a few places the name was adjusted. E.g. in Compositor
the factory method was called createCompositor instead of create.

REVIEW: 109865