Fixed parent linking and root parent linking (#72)

Fixed child folder setting to the highest level root folder, also fixed linking between parent folder and child project in the generated sln file as this was missing in some cases

src/SlnGen.Build.Tasks.UnitTests/SlnFileTests.cs

@@ -60,6 +60,31 @@ public void NoFolders()
             ValidateProjectInSolution((s, p) => p.ParentProjectGuid.ShouldBe(null), projects, false);
         }
 
+        [Fact]
+        public void WithFolders()
+        {
+            SlnProject[] projects =
+            {
+                new SlnProject(GetTempFileName(), "ProjectA", new Guid("C95D800E-F016-4167-8E1B-1D3FF94CE2E2"), new Guid("88152E7E-47E3-45C8-B5D3-DDB15B2F0435"), new[] { "Debug" }, new[] { "x64" }, isMainProject: true, isDeployable: false),
+                new SlnProject(GetTempFileName(), "ProjectB", new Guid("EAD108BE-AC70-41E6-A8C3-450C545FDC0E"), new Guid("F38341C3-343F-421A-AE68-94CD9ADCD32F"), new[] { "Debug" }, new[] { "x64" }, isMainProject: false, isDeployable: false),
+                new SlnProject(GetTempFileName(), "ProjectC", new Guid("00C5B0C0-E19C-48C5-818D-E8CD4FA2A915"), new Guid("F38341C3-343F-421A-AE68-94CD9ADCD32F"), new[] { "Debug" }, new[] { "x64" }, isMainProject: false, isDeployable: false),
+            };
+
+            var action = new Action<SlnProject, ProjectInSolution>((s, p) =>
+            {
+                if (s.IsMainProject)
+                {
+                    p.ParentProjectGuid.ShouldBeNull();
+                }
+                else
+                {
+                    p.ParentProjectGuid.ShouldNotBeNull();
+                }
+            });
+
+            ValidateProjectInSolution(action, projects, true);
+        }
+
         [Fact]
         public void SaveToCustomLocationCreatesDirectory()
         {

src/SlnGen.Build.Tasks/Internal/SlnHierarchy.cs

@@ -145,6 +145,7 @@ private void CreateHierarchy(SlnProject project)
             if (!_rootFolder.Folders.Contains(childFolder))
             {
                 _rootFolder.Folders.Add(childFolder);
+                childFolder.Parent = _rootFolder;
             }
         }