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/services/scripting/scriptingService.ts

@@ -25,6 +25,11 @@ export interface IScriptingService {
 	 */
 	registerProvider(providerId: string, provider: sqlops.ScriptingProvider): void;
 
+	/**
+	 * Specifies whether a provider with a given ID has been registered or not
+	 */
+	isProviderRegistered(providerId: string): boolean;
+
 	/**
 	 * Callback method for when scripting is complete
 	 */
@@ -99,6 +104,11 @@ export class ScriptingService implements IScriptingService {
 		this._providers[providerId] = provider;
 	}
 
+	public isProviderRegistered(providerId: string): boolean {
+		let provider = this._providers[providerId];
+		return !!provider;
+	}
+
 	public dispose(): void {
 		this.disposables = dispose(this.disposables);
 	}