Ground work for Settings/Preferences

Core interface to expose toggling some of the WebPreferences
for now and most probably some of the WebRuntimeFeatures soon.

The whole dummy settings business is meant to keep things
from breaking too much when bisecting, because it is assumed
that there are always valid settings for a given adapterClient.

Change-Id: Ic0a62bcb5af8c0254436dc770b43cde5016c3bbd
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>