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

samples/sqlservices/src/extension.ts

@@ -0,0 +1,37 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as vscode from 'vscode';
+
+import MainController from './controllers/mainController';
+
+let mainController: MainController;
+
+export function activate(context: vscode.ExtensionContext): Promise<boolean> {
+	let activations: Promise<boolean>[] = [];
+
+	// Start the main controller
+	mainController = new MainController(context);
+	context.subscriptions.push(mainController);
+	activations.push(mainController.activate());
+
+	return Promise.all(activations)
+		.then((results: boolean[]) => {
+			for (let result of results) {
+				if (!result) {
+					return false;
+				}
+			}
+			return true;
+		});
+}
+
+export function deactivate(): void {
+	if (mainController) {
+		mainController.deactivate();
+	}
+}