support disabling the modelObserver multiple times
support disabling the modelObserver multiple times

you have to call enable() as often as disable() to enable it again.
This is useful because ProxyModels would enable a disabled observer.

darimpulso/koala-framework,koala-framework/koala-framework,koala-framework/koala-framework,nsams/koala-framework,Ben-Ho/koala-framework,kaufmo/koala-framework,kaufmo/koala-framework,fraxachun/koala-framework,nsams/koala-framework,Ben-Ho/koala-framework,Sogl/koala-framework,nsams/koala-framework,fraxachun/koala-framework,kaufmo/koala-framework,yacon/koala-framework,yacon/koala-framework,yacon/koala-framework,darimpulso/koala-framework,Sogl/koala-framework