Breaking (but minor) changes to the ValueTree::Listener callback methods

Just a quick note to justify my changes to the ValueTree::Listener::valueTreeChildRemoved() and valueTreeChildOrderChanged() methods, which add a couple of extra parameters and will mean that you'll need to make a minor tweak if you've used these classes yourself.

I was trying to create a class of functionality that was just completely impossible without this extra info, so reluctantly decided to modify the method parameters to include the missing info.

FYI, the changes you'll need to make are very minor, e.g.

    void valueTreeChildRemoved (ValueTree&, ValueTree&) override;
    void valueTreeChildOrderChanged (ValueTree&) override;


    void valueTreeChildRemoved (ValueTree&, ValueTree&, int oldIndex) override;
    void valueTreeChildOrderChanged (ValueTree&, int oldIndex, int newIndex) override;