Upgrade to netstandard 2.0. remove unused files

[asizikov]

Breaking change:

ColorSerializer removed, because it depends on System.Drawing APIs that were removed from netstandard

[tumtumtum]

Sorry, I didn't read all the changes but what replaces src/Platform.Xml.Serialization/Attributes/XmlSerializationAttribute.cs etc in netstandard?

[asizikov]

huh, good question. Yeah, they were auto-removed as unused classed. My bad, I'll review and revert that

[asizikov]

@tumtumtum ok, I compared with the original version, and I can confirm that deleted attributes and type serializers files where not in use. /Attributes and /TypeSerializers dirs were not a part of Platform.Xml.Serialization solution. Same attributes are still in the solution, but they are placed according to "one class per file" rule.

For example XmlCDataAttribute is still there: https://github.com/asizikov/Platform/blob/feature/netstandard/src/Platform.Xml.Serialization/XmlCDataAttribute.cs

[asizikov]

@tumtumtum did you have time to have another look?