This was originated for #1279 (CLI-only Aseprite) which can be
achieved with LAF_BACKEND=none anyway.
In this way we simplify the development process, and checking for the
availability of the GUI can be done in run-time through App::isGui()
or Context::isUIAvailable().
If the current UndoState doesn't modify the "saved state" (e.g. there
is a sequence of undoes/redoes that doesn't modify the saved version
of the sprite compared to the current one), we can indicate that we
are in the saved state anyway (!Doc::isModified).
Some extra changes introduced:
* DocUndo & CmdTransaction were simplified: removing the saved
counter, and storing a specific UndoState pointing to the state that
matches the version in the disk
* DocUndo::onDeleteUndoState() can generate a
impossibleToBackToSavedState() if the saved state is deleted. This
might fix some bugs where a "save changes" dialog weren't displayed
after undoing and making changes (probably related to #3542, but not
sure)
Some extra work is needed to avoid showing the "save changes" dialog
if we are close to the saved state and only non-modification undo
states separate us from there. E.g. if we open a file, select the
canvas, and close it, Aseprite now shows the "save changes" dialog,
this wasn't true in previous versions.
This is a first step to include the current selected range of layers
and frames inside the doc::Site structure. So in the future we'll not need
to know about the Timeline in commands that handle those ranges.
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
