Spreadsheet Fixes

-Fix ArithmeticExpression + and - String cell value evaluation to check
for number properly.
-decodePossibleCellAbsolute by location rather than decodeCellAbsolute
in "getPossibleCellEditor" to avoid error being thrown unnecessarily
when trying to access cells by ID "Headers" (which is not a proper cell
ID)
-Update cell content expression after change rather than leaving the
default value.

gitlab #43
gitlab #44
gitlab #45

Change-Id: I8a89d86f6cdfd0651a36176a69066b8faa4faeac