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/connection/common/connectionStore.ts

@@ -240,7 +240,7 @@ export class ConnectionStore {
 				return connectionProfile;
 			} else {
 				return undefined;
-			};
+			}
 		});
 	}
 
@@ -352,7 +352,7 @@ export class ConnectionStore {
 		list.unshift(savedProfile);
 
 		let newList = list.map(c => {
-			let connectionProfile = c ? c.toIConnectionProfile() : undefined;;
+			let connectionProfile = c ? c.toIConnectionProfile() : undefined;
 			return connectionProfile;
 		});
 		return newList.filter(n => n !== undefined);
@@ -372,7 +372,7 @@ export class ConnectionStore {
 		});
 
 		let newList = list.map(c => {
-			let connectionProfile = c ? c.toIConnectionProfile() : undefined;;
+			let connectionProfile = c ? c.toIConnectionProfile() : undefined;
 			return connectionProfile;
 		});
 		return newList.filter(n => n !== undefined);