reset timestamp to 1. This led to a bug where e.g. deleting painted facets
through the respective item in object list followed by possible other actions
and undo restored the painted facets from the time when the project was loaded.
I'm not sure if there was any other situation where this problem manifested.
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.
If an object indicates a valid timestamp, then the timestamp
is relied upon to not serialize the object data if the timestamp
of the same object on the undo/redo stack matches.
