Warnings are shown, but features can be committed. Fields which
fail an unenforced constraint are now shaded in yellow to differentiate
from the red failure for enforced constraints.
and not any constraints enforced by the provider
Because:
1. we want to keep browsing features nice and responsive. It's nice to give
feedback as to whether the value checks out, but not if it's too slow to
do so. Some constraints (eg unique) can be expensive to test. A user can
freely remove a layer-based constraint if it proves to be too slow to
test, but they are unlikely to have any control over provider-side
constraints
2. the provider has already accepted the value, so presumably it doesn't
violate the constraint and there's no point rechecking!
Contains static helper methods for working with vector layers.
Initially only contains a method to test whether a value
exists within a layer's attributes.
