Move type LabelSelector and type LabelSelectorRequirement from pkg/apis/extensions
This avoids an import loop when Job (and later DaemonSet, Deployment, ReplicaSet)
are moved out of extensions to new api groups.

Also Move LabelSelectorAsSelector utility from pkg/apis/extensions/ to pkg/api/unversioned/
Also its test.
Also LabelSelectorOp* constants.
Also the pkg/apis/extensions/validation functions ValidateLabelSelectorRequirement and
ValidateLabelSelector move to pkg/api/unversioned

The related type in pkg/apis/extensions/v1beta1/ is staying there.  I might move
it in another PR if neccessary.