[Feature idea] Add ability to group project for build/test

[PadreSVK]

Hi it is possible to add ability to specify projects groups based on trait|category|group?
This could replace slnf, so instead of having several slnf files in repository we could have only one slnx file where could be projects annotated with props. i.e Category|Group...

Example

<Solution>
  <Configurations>
    <BuildType Name="Debug" />
    <BuildType Name="Release" />
  </Configurations>
  <Folder Name="/Solution Items/">
    <File Path=".editorconfig" />
    <File Path="README.md" />
    <File Path="NuGet.Config" />
    <File Path="Directory.Build.props" />
  </Folder>
  <Folder Name="/test/">
    <Project Path="test\FunctionalTests\FunctionalTests.csproj" />
    <Project Path="test\UnitTests\UnitTests.csproj" Category="UnitTests"/>
    <Project Path="test\UnitTests1\UnitTests1.csproj" Category="UnitTests"/>
  </Folder>
  <Folder Name="/src/">
    <Project Path="src\My.API\My.API.csproj" Category="Group1"/>
    <Project Path="src\My.API1\My.API1.csproj" Category="Group2"/>
    <Project Path="src\My.API2\My.API2.csproj" Category="Group1"/>
    <Project Path="src\My.API3\My.API3.csproj" Category="Group1"/>
    <Project Path="src\My.API4\My.API4.csproj" Category="Group2"/>
    <Project Path="src\My.API5\My.API5.csproj" Category="Group1"/>
  </Folder>
</Solution>

Usage

1. dotnet test --filter Category=UnitTests
   this should build&test test\UnitTests\UnitTests.csproj and test\UnitTests1\UnitTests1.csproj
2. dotnet build --filter Category=Group2
   this should build src\My.API1\My.API1.csproj and src\My.API4\My.API4.csproj

[PadreSVK]

This issue could be merged with #85