A tab's disposition can be foreground (e.g. target="_blank") or background
(Ctrl+Click, or middle-click). API consumers should be able to discern between
the two intents, so they can decide whether or not to focus their new view.

Change-Id: Iab3ccb9e9f0b85256892f4f74204515bdeefa695
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>