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/base/browser/ui/panel/tab.component.ts

@@ -29,19 +29,22 @@ export class TabComponent implements OnDestroy {
 	@Input() public identifier: string;
 	@Input() private visibilityType: 'if' | 'visibility' = 'if';
 	private rendered = false;
+	private destroyed: boolean = false;
 
 	constructor(
 		@Inject(forwardRef(() => ChangeDetectorRef)) private _cd: ChangeDetectorRef
 	) { }
 
 	public set active(val: boolean) {
-		this._active = val;
-		if (this.active) {
-			this.rendered = true;
-		}
-		this._cd.detectChanges();
-		if (this.active && this._child) {
-			this._child.layout();
+		if (!this.destroyed) {
+			this._active = val;
+			if (this.active) {
+				this.rendered = true;
+			}
+			this._cd.detectChanges();
+			if (this.active && this._child) {
+				this._child.layout();
+			}
 		}
 	}
 
@@ -50,6 +53,7 @@ export class TabComponent implements OnDestroy {
 	}
 
 	ngOnDestroy() {
+		this.destroyed = true;
 		if (this.actions && this.actions.length > 0) {
 			this.actions.forEach((action) => action.dispose());
 		}