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/controllers/splitPropertiesPanel.ts

@@ -0,0 +1,43 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 sqlops from 'sqlops';
+
+/**
+ * The main controller class that initializes the extension
+ */
+export default class SplitPropertiesPanel {
+	private panels: sqlops.FlexContainer[];
+	private _modelBase: sqlops.FlexContainer;
+	constructor(view: sqlops.ModelView, numPanels: number) {
+		this.panels = [];
+		let ratio = Math.round(100 / numPanels);
+		for (let i = 0; i < numPanels; i++) {
+			this.panels.push(view.modelBuilder.flexContainer()
+				.withLayout({ flexFlow: 'column' }).component());
+		}
+		this._modelBase = view.modelBuilder.flexContainer()
+			.withLayout({
+				flexFlow: 'row'
+			}).withItems(this.panels, {
+				flex: `0 1 ${ratio}%`
+			})
+			.component();
+	}
+
+	public get modelBase(): sqlops.Component {
+		return this._modelBase;
+	}
+
+	public addItem(item: sqlops.Component, panel: number): void {
+		if (panel >= this.panels.length) {
+			throw new Error(`Cannot add to panel ${panel} as only ${this.panels.length - 1} panels defined`);
+		}
+		this.panels[panel].addItem(item, undefined);
+	}
+}
+