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- 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