Kill/Shutdown multiple apps from AppsList

Fixes [MLHR-1407](https://malhar.atlassian.net/browse/MLHR-1407) and [MLHR-1408](https://malhar.atlassian.net/browse/MLHR-1408). Also adds a `sprintf` method to dtText.

NOTE: The user-experience for killing apps will not get better until the gateway can provide the "shutting down" status of an app over WebSocket/REST