Feat/model backed ui (#1145)

This is an initial PR for a new model-driven UI where extensions can provide definitions of the components & how they're laid out using Containers.
#1140, #1141, #1142, #1143 and #1144 are all tracking additional work needed to improve the initial implementation and fix some issues with the implementation.

Features:
- Supports defining a FlexContainer that maps to a flexbox-based layout.
- Supports creating a card component, which is a key-value pair based control that will lay out simple information to a user. Eventually this will have an optional set of actions associated with it.
- Has a sample project which shows how to use the API and was used for verification

src/sql/workbench/api/node/sqlExtHost.contribution.ts

@@ -19,6 +19,7 @@ import 'sql/workbench/api/node/mainThreadResourceProvider';
 import 'sql/workbench/api/electron-browser/mainThreadTasks';
 import 'sql/workbench/api/electron-browser/mainThreadDashboard';
 import 'sql/workbench/api/node/mainThreadDashboardWebview';
+import 'sql/workbench/api/node/mainThreadModelView';
 import './mainThreadAccountManagement';
 import { LifecyclePhase } from 'vs/platform/lifecycle/common/lifecycle';