Sometimes the user expresses how they want a link to be opened, eg "open in new tab" or pasting a link into an existing window, but other times it's ambiguous, such as clicking a link or tapping a notification that subsequently calls openWindow.

This should allow developers to create single and multi-document applications by determining how links are handled.

Some patterns to enable:

User clicks on a link in a native app, developer wants it to launch in a new chromeless window (the user has already added this site to homescreen).

User clicks on a link on a website, developer wants to focus an existing client and target the navigation at it.

User clicks on a notification, developer wants to focus an existing client, discard the navigation, and postmessage the client.

User clicks on a homescreen icons, developer wants to create a new chromeless client, even though there's an already-open client for the target url.