Fix data validation issues (#975)

* Fix `SetDropList` to allow XML special characters

* This closes #971, allow quotation marks in SetDropList()

This patch included a XML entity mapping table instead of
xml.EscapeText() to be fully compatible with Microsoft Excel.

* This closes #972, allow more than 255 bytes of validation formulas

This patch changed the string length calculation unit of data
validation formulas from UTF-8 bytes to UTF-16 code units.

* Add unit tests for SetDropList()

* Fix: allow MaxFloat64 to be used in validation range

17 decimal significant digits should be more than enough to represent
every IEEE-754 double-precision float number without losing precision,
and numbers in this form will never reach the Excel limitation of 255
UTF-16 code units.

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