Allow installation to the @thumbs folders

fixes #1404

Core/ModuleInstaller.cs

@@ -442,6 +442,15 @@ internal static List<InstallableFile> FindInstallableFiles(ModuleInstallDescript
                     case "Ships/SPH":
                         installDir = ksp == null ? null : ksp.ShipsSph();
                         break;
+                    case "Ships/@thumbs":
+                        installDir = ksp == null ? null : ksp.ShipsThumbs();
+                        break;
+                    case "Ships/@thumbs/VAB":
+                        installDir = ksp == null ? null : ksp.ShipsThumbsVAB();
+                        break;
+                    case "Ships/@thumbs/SPH":
+                        installDir = ksp == null ? null : ksp.ShipsThumbsSPH();
+                        break;
                     default:
                         throw new BadInstallLocationKraken("Unknown install_to " + stanza.install_to);
                 }

Spec.md

@@ -274,7 +274,7 @@ three source directives:
 In addition a destination directive *must* be provided:
 
 - `install_to`: The location where this section should be installed.
-  Valid values for this entry are `GameData`, `Ships`, `Ships/SPH`(**v1.12**), `Ships/VAB`(**v1.12**), `Tutorial`, `Scenarios` (**v1.14**)
+  Valid values for this entry are `GameData`, `Ships`, `Ships/SPH`(**v1.12**), `Ships/VAB`(**v1.12**), `Ships/@thumbs/VAB`(**v1.16**), `Ships/@thumbs/SPH`(**v1.16**), `Tutorial`, `Scenarios` (**v1.14**)
   and `GameRoot` (which should be used sparingly, if at all).
   Paths will be preserved, but directories will *only*
   be created when installing to `GameData`, `Tutorial`, or `Scenarios`.

Tests/Core/ModuleInstaller.cs

@@ -473,6 +473,9 @@ public void ModuleManagerInstancesAreDecoupled()
         [TestCase("Ships")]
         [TestCase("Ships/VAB")]
         [TestCase("Ships/SPH")]
+        [TestCase("Ships/@thumbs")]
+        [TestCase("Ships/@thumbs/VAB")]
+        [TestCase("Ships/@thumbs/SPH")]
         public void AllowsInstallsToShipsDirectories(string directory)
         {
             // Arrange