We should either only do this for browser windows (not app windows where the mozapp attribute is set), or figure out how to handle results for installed apps.

We can explore the latter in the search app, but one option is to assume that a URL from the same origin as an app manifest belongs to that app (as it's one app per origin and one origin per app). Then we can open app URLs in the correct open or new app window by launching a new app window with the correct manifest URL, and open non-app URLs in browser windows.

When sheets come along we can group the sheets for an app by the app's origin and all other sheets under the catch-all "browser" group.