Refactor custom views:

- Custom view service to get tree item viewer
- Custom view service to register the data provider

src/vs/workbench/api/browser/viewsExtensionPoint.ts

@@ -8,7 +8,7 @@ import { localize } from 'vs/nls';
 import { forEach } from 'vs/base/common/collections';
 import { IJSONSchema } from 'vs/base/common/jsonSchema';
 import { ExtensionMessageCollector, ExtensionsRegistry } from 'vs/platform/extensions/common/extensionsRegistry';
-import { ViewLocation, ViewsRegistry, IViewDescriptor } from 'vs/workbench/common/views';
+import { ViewLocation, ViewsRegistry, ICustomViewDescriptor } from 'vs/workbench/common/views';
 import { CustomTreeViewPanel } from 'vs/workbench/browser/parts/views/customView';
 import { ContextKeyExpr } from 'vs/platform/contextkey/common/contextkey';
 import { coalesce, } from 'vs/base/common/arrays';
@@ -102,14 +102,15 @@ ExtensionsRegistry.registerExtensionPoint<{ [loc: string]: schema.IUserFriendlyV
 				const registeredViews = ViewsRegistry.getViews(location);
 				const viewIds = [];
 				const viewDescriptors = coalesce(entry.value.map(item => {
-					const viewDescriptor = <IViewDescriptor>{
+					const viewDescriptor = <ICustomViewDescriptor>{
 						id: item.id,
 						name: item.name,
 						ctor: CustomTreeViewPanel,
 						location,
 						when: ContextKeyExpr.deserialize(item.when),
 						canToggleVisibility: true,
-						collapsed: true
+						collapsed: true,
+						treeItemView: true
 					};
 
 					// validate