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

.vscode/launch.json

@@ -89,7 +89,8 @@
 			"skipFiles": [
 				"**/winjs*.js"
 			],
-			"webRoot": "${workspaceFolder}"
+			"webRoot": "${workspaceFolder}",
+			"timeout": 15000
 		},
 		{
 			"type": "node",