Found no fewer than three separate places that cause a pop-up prompt

for saving on a cell which is completely unmodified.  One of these
was due to the BPlane implementation, which forces an instance to be
deleted and re-placed on a bounding box recomputation, which should
not, in that case, be considered a modification.  Another always runs
DRC on a subcell upon reading rather than relying on any checkplane
entries in the file itself;  and the last one marking the timestamp
as modified stemming from an attempt to correct an O(N^2) check to
O(N).  All three cases have now been corrected.

database/database.h.in

@@ -790,7 +790,9 @@ extern CellUse *DBFindUse();
 
     /* Insertion/deletion of cell uses into the cell tile plane of a parent */
 extern void DBPlaceCell();
+extern void DBPlaceCellNoModify();
 extern void DBDeleteCell();
+extern void DBDeleteCellNoModify();
 extern void DBClearCellPlane();
 
     /* Insertion/deletion of cell uses into the name space of a parent */