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/sqlops.proposed.d.ts

@@ -973,4 +973,31 @@ declare module 'sqlops' {
 		 */
 		export function getProvidersByType<T extends DataProvider>(providerType: DataProviderType): T[];
 	}
+
+	/**
+	 * Context object passed as an argument to command callbacks.
+	 * Defines the key properties required to identify a node in the object
+	 * explorer tree and take action against it.
+	 */
+	export interface ObjectExplorerContext {
+
+		/**
+		 * The connection information for the selected object.
+		 * Note that the connection is not guaranteed to be in a connected
+		 * state on click.
+		 */
+		connectionProfile: IConnectionProfile;
+		/**
+		 * Defines whether this is a Connection-level object.
+		 * If not, the object is expected to be a child object underneath
+		 * one of the connections.
+		 */
+		isConnectionNode: boolean;
+		/**
+		 * Node info for objects below a specific connection. This
+		 * may be null for a Connection-level object
+		 */
+		nodeInfo: NodeInfo;
+	}
+
 }