New class ModelConfig wrapping DynamicPrintConfig and a timestamp

to help with detecting "not changed" event when taking
Undo/Redo snapshot or synchronizing with the back-end.

Converted layer height profile and supports / seam painted areas
to the same timestamp controlled structure.

src/libslic3r/ObjectID.cpp

@@ -17,6 +17,8 @@ ObjectID wipe_tower_instance_id()
     return mine.id();
 }
 
+size_t ObjectWithTimestamp::s_last_timestamp = 1;
+
 } // namespace Slic3r
 
 // CEREAL_REGISTER_TYPE(Slic3r::ObjectBase)