Need support globs outside of the project cone

By default, project items outside of the project cone are not shown in the solution explorer, unless they have link metadata. To show files added through glob, the developer must add globs like this inside the project:

   <Compile Include="..\Shared\**\*.cs">
      <Link>ExternalFiles\%(RecursiveDir)\%(Filename)%(Extension)</Link>
    </Compile>

But this is hard to remember and hard to use. The idea is to create something in the .targets file, and apply Link metadata to items with <LinkBase> metadata, so inside the project, it can be:

   <Compile Include="..\Shared\**\*.cs">
      <LinkBase>ExternalFiles</LinkBase>
    </Compile>

or

   <Compile Include="..\Shared\**\*.cs">
      <LinkBase></LinkBase>
    </Compile>

If it maps to the project root folder directly.

[davkean]

@nguerrera The latter.

[srivatsn]

Fixed by #1246