+ Added "undo" library (undo namespace).
+ Moved UndoHistory to undo namespace.
+ Added Undoer and UndoersCollector interfaces.
+ Converted old UndoStream to UndoersStack class.
+ Added new "undoers" namespace and moved each Undoer implementation
(classes to undo different actions) to "undoers" directory.
+ Added IO operations to "raster" objects (image, cel, layer, palette,
mask).