Extensibility: Context menu support in Object Explorer (#1883)

- Fixes #1867 context menu should be extensible
- Added context keys to support "when" conditions on the new extensions
- Fixes issue where actions like New Query, scripting show up even if these are not valid for the provider type or object type
- Fixed node expansion bug where rapid connect / expand / disconnect could break the app (fix in ObjectExplorerService.onNodeExpanded)
- Major change to how internal actions work. These cannot assume the context has non-serializable objects. Opened up some APIs to make this easier to handle.
- Fixed a number of existing bugs in internal actions.
  - Notably, DisconnectAction was adding a listener on each right-click on an active connection and never getting it disposed. This wasn't needed at all due to design changes.
  - Another bug fix is that the Manage action now correctly navigates to the DB dashboard for database-level connections. Before this it went to the server-level dashboard.

* Define API for context info

src/sql/parts/query/common/queryManagement.ts

@@ -21,6 +21,7 @@ export interface IQueryManagementService {
 	_serviceBrand: any;
 
 	addQueryRequestHandler(queryType: string, runner: IQueryRequestHandler): IDisposable;
+	isProviderRegistered(providerId: string): boolean;
 	registerRunner(runner: QueryRunner, uri: string): void;
 
 	cancelQuery(ownerUri: string): Thenable<sqlops.QueryCancelResult>;
@@ -82,7 +83,6 @@ export interface IQueryRequestHandler {
 }
 
 export class QueryManagementService implements IQueryManagementService {
-	public static readonly DefaultQueryType = 'MSSQL';
 	public _serviceBrand: any;
 
 	private _requestHandlers = new Map<string, IQueryRequestHandler>();
@@ -143,6 +143,11 @@ export class QueryManagementService implements IQueryManagementService {
 		};
 	}
 
+	public isProviderRegistered(providerId: string): boolean {
+		let handler = this._requestHandlers.get(providerId);
+		return !!handler;
+	}
+
 	private addTelemetry(eventName: string, ownerUri: string, runOptions?: sqlops.ExecutionPlanOptions): void {
 		let providerId: string = this._connectionService.getProviderIdFromUri(ownerUri);
 		let data: TelemetryUtils.IConnectionTelemetryData = {