Undo System: remove accumulate/store modes

This complicated handling of undo steps in a generic way
especially switching between undo systems that stored data to ones
that accumulated changes.

Now each undo system must treat it's steps as check-point,
internally it can apply/rewind changes.

This commit also fixes projection paint where the object mode wasn't
following the undo steps.

source/blender/editors/physics/particle_edit_undo.c

@@ -283,7 +283,6 @@ void ED_particle_undosys_type(UndoType *ut)
 
 	ut->step_foreach_ID_ref = particle_undosys_foreach_ID_ref;
 
-	ut->mode = BKE_UNDOTYPE_MODE_STORE;
 	ut->use_context = true;
 
 	ut->step_size = sizeof(ParticleUndoStep);