Add VideoDecoder internal callbacks (e.g. to stop the video at the end)
Add SliderGUI::getIndex() method.

It allows to know the index of the slide we're in.

It's only meaningful with SliderBlockGUI, but since some compilation
problems with the rest of the project, it was better to simply create
it as a virtual method in the base class (SliderGUI).
Add RectGUI::rotate method
Add VideoDecoder::{maximize, restore} methods (not working properly yet)
Add VideoDecoder auxiliar methods
Add SliderGUI::getEnabled() method