Undoers were little objects to swap/revert an action. They didn't
execute the action itself, they just revert its previous state. Now
undoers were replaced with cmds: A cmd is an object that
executes/undoes/redoes just one action.
Changes:
* Remove old undo library and app/objects_container_impl.cpp
(now we use the doc::ObjectId directly to store undo info)
* Remove all Undoers from app/undoers/
* Replace DocumentApi impl with little Cmds in app/cmd/, these
cmds handle execute/undo/redo of each action at the logic layer
* Remove doc::Dirty object
* Remove doc::Settings: all undo configuration is in the app side
* Move undo options from app:ISettings to app::Preferences
* Rename UndoTransaction to Transaction
* Create a CmdSequence to store a sequence of Cmds (as now the new
undo library doesn't support open/close groups)
* Add doc::get<T>(ObjectId) function to get any kind of object
from the doc library by its ID
* Add Cel::document() and Sprite::document() members
* Add Sprite::cels(frame_t) to get all cels in the given frame
* Add Layer::displaceFrames() member function
* Move the "allow non-linear history" flag from undo2::UndoHistory
to app::DocumentUndo
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
Changes:
* Create render library (move util/render.cpp to render/render.cpp)
* Move app::Zoom class to render::Zoom
* Remove doc::Image::merge() member function
* Add gfx::Clip helper class (to clip dst/src rectangles before a blit)
* Move doc::composite_image() to render::composite_image()
* Remove doc::Sprite::render()
* Replace Sprite::getPixel() with render::get_sprite_pixel()
* Remove doc::layer_render() function
* Convert DitheringMethod to a enum class
* Add AppRender to configure a render::Render with the app configuration
* Move checked background preferences as document-specific configuration
* Add doc::Sprite::layer() and palette() member functions
* Add doc::Layer::cel() member function
* Add doc::Palette::entry() member function()
* Add doc::frame_t type
* Move create_palette_from_rgb/convert_pixel_format to render library
* ExportSpriteSheet doesn't need a temporary image now that we can specify
the source rectangle in the render routine
