Writes value into cell.
false when the cell refuses the write: it holds a formula, or it is locked on a
protected sheet (the user is told why).
Writes value into every cell of range that holds no formula, in one operation.
false when nothing was written: every cell holds a formula, or the sheet refused
(a locked cell on a protected sheet; the user is told why).
What a cell content entry writes through: guarded writes into the sheet, and the notifier for anything else it has to say. One per layer; every entry of the layer shares it.