Export maximizeMode to scripting

Export maximizeMode as a property, it's important as scripts need to go trough some hoops to know whether a window is maximized or not (connect to maximizeModeAboutToChange which has the mode as parameter and then store it until maximizeChanged is emitted)

Since maximizing is a multi step transaction, the property is read only to set maximized from scripting the invokable setMaximize() should be used instead.