... with proper tools::Date methods Normalize() and AddMonths().

Also prepare functionality to easily switch on rollover for StarBASIC as well,
i.e. when called by DateSerial() runtime function.

For StarBASIC, invalid date for day > daysinmonthofyear is now (or better since
a previous commit 94bb96ada421b423e9ed30526fe5a6aac95f00b9 from today) properly
detected, not just dumb 1<=day<=31.

Change-Id: Ibb44f7247726f1e1168f0e66c5ae18e073d19f08