Flatten GUI tree somewhat

The very deep tree structure didn't really aggree with me, sorry. This
makes the core module rather large, but on the other hand that just
highlights that it is rather large.

gui/syncthing/core/uniqueFolderDirective.js

@@ -0,0 +1,21 @@
+angular.module('syncthing.core')
+    .directive('uniqueFolder', function () {
+        return {
+            require: 'ngModel',
+            link: function (scope, elm, attrs, ctrl) {
+                ctrl.$parsers.unshift(function (viewValue) {
+                    if (scope.editingExisting) {
+                        // we shouldn't validate
+                        ctrl.$setValidity('uniqueFolder', true);
+                    } else if (scope.folders.hasOwnProperty(viewValue)) {
+                        // the folder exists already
+                        ctrl.$setValidity('uniqueFolder', false);
+                    } else {
+                        // the folder is unique
+                        ctrl.$setValidity('uniqueFolder', true);
+                    }
+                    return viewValue;
+                });
+            }
+        };
+});