Add button to launch app from install notification

[cassidyjames]

Problem

If you kick off an app install and then navigate away from AppCenter, you get a notification when it's done installing but can't immediately open the app; you have to either look in the Applications Menu or go back to AppCenter and hit the Open button.

Proposal

Add a button to open the app in the installation success notification.

Prior Art

GNOME Software does exactly this.