Add a way to construct a query using flags

This allows us to pseudo-dynamically construct a query from prepared
"features", so each client app can ask for things it really wants/needs,
for example only contacts with email addresses, only IM contacts with
avatars etc. Another set of flags sets optional features (equals to
OPTIONAL {...} in SparQL), which retrieves contacts that both have and
not have the selected feature, eg. avatars (some contacts don't have
avatars, but we still want to have them in the list).

REVIEW: 109833