Undo: unified undo system w/ linear history

- 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

source/blender/editors/include/ED_sculpt.h

@@ -36,6 +36,9 @@ struct Object;
 struct RegionView3D;
 struct ViewContext;
 struct rcti;
+struct UndoStep;
+struct UndoType;
+struct ListBase;
 
 /* sculpt.c */
 void ED_operatortypes_sculpt(void);
@@ -43,4 +46,7 @@ void ED_sculpt_redraw_planes_get(float planes[4][4], struct ARegion *ar,
                                  struct RegionView3D *rv3d, struct Object *ob);
 int  ED_sculpt_mask_box_select(struct bContext *C, struct ViewContext *vc, const struct rcti *rect, bool select, bool extend);
 
+/* sculpt_undo.c */
+void ED_sculpt_undosys_type(struct UndoType *ut);
+
 #endif /* __ED_SCULPT_H__ */