Created by: patloew
Added callbacks to PanelSlideListener, for when the panel state starts changing from collapsed to expanded and vice versa. This can be useful, if other UI elements need to start changing already when the state transition begins.
Added StartPanelSlideListener, which behaves like the normal PanelSlideListener for collapse and expand events, except that it executes on slide start and automatically handles the case when state change does not finish by calling the other callback again.