Apply https://closedxml.codeplex.com/SourceControl/network/forks/NickNack2020/ClosedXml/contribution/8268 : ...

Possibly fixes #63

Added XML encoder util. Will encode invalid XML characters based on
XML Spec http://www.w3.org/TR/REC-xml/#charsets

-Encode shared string on save
-Decoded shared strings on load

Quick fix only applied to shared strings,
could be other problematic areas in code

# Conflicts:
#	ClosedXML_Tests/ClosedXML_Tests.csproj
#	ClosedXML_Tests/Excel/Cells/XLCellTests.cs

Project hierarchy cleanup