- Undo that changes modes currently asserts,
since undo is now screen data.
Most likely we will change how object mode and workspaces work
since it's not practical/maintainable at the moment.
- Removed view_layer from particle settings
(wasn't needed and complicated undo).
- Use a single undo history for all operations.
- UndoType's are registered and poll the context to check if they
should be used when performing an undo push.
- Mode switching is used to ensure the state is correct before
undo data is restored.
- Some undo types accumulate changes (image & text editing)
others store the state multiple times (with de-duplication).
This is supported by checking UndoStack.mode `ACCUMULATE` / `STORE`.
- Each undo step stores ID datablocks they use with utilities to help
manage restoring correct ID's.
Needed since global undo is now mixed with other modes undo.
- Currently performs each undo step when going up/down history
Previously this wasn't done, making history fail in some cases.
This can be optimized to skip some combinations of undo steps.
grease-pencil is an exception which has not been updated
since it integrates undo into the draw-session.
See D3113
It was too tricky to know ahead of time if an object would still
be visible in the new window/workspace/scene/layer combination,
especially since other windows may share some of these data-blocks.
So store the context, make the change, then check if the object is
still visible, freeing mode data of it's not.
