Changes:
* Add DataRecoveryView
* Split SubObjectsIO into an interface and SubObjectsFromSprite
* Add a "setId" parameter to some doc::read_* functions (we need this
functionality from session recovery)
Changes:
* Add doc::ImageRef to count references to the same image between Cels
(at this moment we cannot generate linked cels anyway)
* Remove doc:Stock class and doc::Sprite::m_stock member variable
* Remove app::undoers::Add/RemoveImage
* Add doc::SubObjectsIO and app::undoers::ObjectIO to
replace doc::LayerSubObjectsSerializer