Move `eUndoStepDir` to `BKE_undo_system.h` and use its values
everywhere.
Note that this also introduce the `STEP_INVALID` value in that enum.
Finally, kept the matching struct members in some lower-level readfile
code as an `int` to avoid having to include `BKE_undo_system.h` in a lot
of unrelated files.
94cf74afbb