Improve color support and more

Merge branch 'develop' of https://github.com/ClosedXML/ClosedXML into netstandard2 ...

# Conflicts:
#	ClosedXML/ClosedXML.csproj
#	ClosedXML/Excel/Drawings/XLPicture.cs
#	ClosedXML/Excel/IXLTheme.cs
#	ClosedXML/Excel/XLWorkbook.cs
#	ClosedXML/Excel/XLWorkbook_Load.cs
#	ClosedXML/Excel/XLWorkbook_Save.cs
#	ClosedXML/Properties/AssemblyInfo.cs
#	ClosedXML/Utils/GraphicsUtils.cs
#	ClosedXML/XLHelper.cs
#	ClosedXML/packages.config
#	ClosedXML_Examples/ClosedXML_Examples.csproj
#	ClosedXML_Examples/Properties/AssemblyInfo.cs
#	ClosedXML_Examples/packages.config
#	ClosedXML_Sandbox/ClosedXML_Sandbox.csproj
#	ClosedXML_Sandbox/Properties/AssemblyInfo.cs
#	ClosedXML_Sandbox/packages.config
#	ClosedXML_Tests/ClosedXML_Tests.csproj
#	ClosedXML_Tests/Properties/AssemblyInfo.cs
#	ClosedXML_Tests/packages.config

Fix line endings of .csproj

Consolidate assembly version info into one file

Upgrade OpenXML to v2.7.2

Convert to Visual Studio 2017 format.

Upgrade OpenXML to v2.7.1

Upgrade to .NET 4.5.2

Load test file in sandbox (#276) ...

* Load test file in sandbox

* Revert to C#5 syntax.

* Add test file

Add .editorconfig file - see http://editorconfig.org/ for its use.

Clean up project files (#26) ...

* Remove SCC tags in projects.

* Correctly reference NuGet dependency.

* Remove dependency on System.Windows.Forms

* Remove all platforms except AnyCPU

* Remove reference to System.Windows.Forms in Tests project

* Remove old-style NuGet references

* Remove DocumentFormat.OpenXml.dll from Solutions. We reference it through NuGet.

* Remove OnBuildSuccess

Remove Code Analysis configurations, which are disabled anyway.

Project hierarchy cleanup