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/tasks/packagetasks.js

@@ -0,0 +1,36 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+let gulp = require('gulp');
+let exec = require('child-process-promise').exec;
+let color = require('gulp-color');
+
+// CONSTANTS ///////////////////////////////////////////////////////////////
+let packageVals = require('../package');
+
+// HELPER FUNCTIONS ////////////////////////////////////////////////////////
+
+let buildPackage = function(packageName) {
+    // Make sure there are
+    if (!packageVals.repository) {
+        return Promise.reject("Repository field is not defined in package.json");
+    }
+
+    // Initialize the package command with program and command
+    let vsceArgs = [];
+    vsceArgs.push('./node_modules/vsce/out/vsce');
+    vsceArgs.push('package'); // package command
+
+    // Add the package name
+    vsceArgs.push('-o');
+    vsceArgs.push(packageName);
+
+    // Put it all together and execute the command
+    let command = vsceArgs.join(' ');
+    console.log(command);
+    return exec(command);
+};