Fix DataRow error management

DataRow's RowError is supposed to be String.Empty to indicate
no errors, but is sometimes wrongly set to null, falsely indicating
errors. Fixed to be String.Empty.
Added test for DataRow error
Test uses SQLite in-memory DB for test

Also spaces to tabs
[monodoc] Changes to support new-style iOS documentation:

- Fixed bug in creating CREF strings.
- Refactored XML file processing for customization during new style iOS assembly. This was done through the new file source interface, which lets you use dependency injection to customize the process.
- Added the ability to rewrite CREF references when assembling docs that use native types. Also, the ability to handle dropped namespaces.
[mdoc] Changes to support new-style iOS API:

- the dropns flag now lets you define a namespace that is dropped in a new version of the assembly during update.
- the ntypes flag lets you tell the update command that certain methods are now using native types, and as such will attempt to match with old types in order to preserve documentation.
- the preserve flag will avoid removing members. This is to avoid dropping documentation for members that changed from one version of the framework to another new-style version (ie. iOS).
[TimeZoneInfo] Fix issue with some floating DST transition times creating invalid DateTime values, and implement ToSerializedString and FromSerializedString.
This change is released under the MIT license.
Test Encoder.Convert and UTF8Encoder.Convert behaviors.
Added class ExposedEncoder to test the virtual method Encoder.Convert behavior.
Test UTF8Encoder.Convert override.
Add test for Regex.Match with anchors on a substring.
Regex fixes for substring parsing.
Miscellaneous changes per review
Add test for Regex RTL substring matching.
Correct code review comments on test
[Cleanup] Removed a bunch of superfluous .csproj and .sln files

Most of these files weren't updated in the last >5 years and serve no purpose today, so they only clutter the repository.
Merge pull request #1197 from akoeplinger/remove-csproj

[Cleanup] Removed a bunch of superfluous .csproj and .sln files
Fixed issue related to fully qualified namespaces being used for XmlEnumAttribute values.
Merge pull request #1198 from chriswebb/master

Fixed issue where XmlReflectionImporter unable to handle XmlEnums with colons
Removed [Category("NotWorking")] from PackUriHelperTests.
CompareInvalidTest is still not working because .NET PackUriHelper.ComparePackUri is doing something that uses Uri and throws an exception whereas Mono is not.
Changed Check.PartUriIsValid to throw ArgumentException.
Added argument validations to ComparePackUri an ComparePartUri.
Merge pull request #1199 from esdrubal/packurihelper

PackUriHelper