Added MonoGame.Content.Builder.targets to automatically build and redirect the output of referenced Content Projects

MonoGame.Content.Builder.targets automatically builds all referenced MonoGame Content Projects ( they need to import a new MonoGame.Content.Pipeline.targets from MonoGame.Framework.Content.Pipeline folder ).

Adding reference to a Content Project is done by creating a new < MonoGameContentReference > item inside the < ItemGroup > tag, for example:

    <ItemGroup>
      <MonoGameContentReference Include="..\Content1\Content1.contentproj" />
    </ItemGroup>

Compiled .xnb files are automatically moved to a game directory. For most platforms it is:

    bin\$(ContentRootDirectory)

For Android:

    obj\assets\$(ContentRootDirectory)

But you can always tweak this.