Add option to open About window first in Single app mode

[cschneebeck]

Add a property to the framework that allows developers to choose if the About window opens first when using Single App Mode. If set to true, users would see the About window instead of the app window when the site loads.

Also add functionality that allows users to override the about window opening first. Developers would add a checkbox to the About window that says something like "Skip on Opening". If checked, a cookie would be stored on the users browser. When a single app site loads, it would check if the user has the cookie to override opening the About window. If it's there, the site would open the app window. If the cookie isn't there, the site would check the property listed above and open the About or app window accordingly.