setEnabled(bool) and setDisabled(bool) should have default parameter values for use with clicked() signals (and the likes)

setEnabled(bool) and setDiabled(bool) do not have default arguments. It is, however, natural to consider that setEnabled() would enable the widget and setDisabled() would disable the widget.

In addition, this prevents using these two slots with signals without arguments, such as "clicked()" and "triggered()".

It is a small changed that has no backward compatibility issues.