Affects a lot of classes. Very thoroughly checked signal connections and deferred calls to this method, add_do_method/add_undo_method calls, and so on.
Also renames the internal `_update_callback()` to `_redraw_callback()` for consistency.
Just a few comments have also been changed to say "redraw".
In CPUParticles2D, there was a private variable with the same name. It has been renamed to `do_redraw`.
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