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
